ESP-IDF fix for cellular HTTP: one more linker workaround (#860)

Commit bf8cd21 introduced a mechanism for leaving out one or more of cell, GNSS or short-range [BLE and Wifi] from a build.  This hit bug in the ESP-IDF linker which means that, if a file happens to contain functions which ALL have WEAK counterparts, the linker will entirely ignore the .c file and use the WEAK versions instead; this is counter-productive, resulting in everything returning "NOT SUPPORTED".

This linker bug was worked-around in commit 6eff2c4 but it turns out that even that missed a bit, in the cellular HTTP code, where the file system calls needed a dummy "link" function to get them included.  Hopefully this is the last of them.

cell/api/u_cell_file.h

@@ -52,6 +52,18 @@ extern "C" {
  * TYPES
  * -------------------------------------------------------------- */
 
+/* ----------------------------------------------------------------
+ * FUNCTIONS:  WORKAROUND FOR LINKER ISSUE
+ * -------------------------------------------------------------- */
+
+/** Workaround for Espressif linker missing out files that
+ * only contain functions which also have weak alternatives
+ * (see https://www.esp32.com/viewtopic.php?f=13&t=8418&p=35899).
+ *
+ * You can ignore this function.
+ */
+void uCellFilePrivateLink(void);
+
 /* ----------------------------------------------------------------
  * FUNCTIONS
  * -------------------------------------------------------------- */

cell/src/u_cell.c

@@ -177,6 +177,7 @@ int32_t uCellInit()
     uCellHttpPrivateLink();
     uCellLocPrivateLink();
     uCellMuxPrivateLink();
+    uCellFilePrivateLink();
 
     if (gUCellPrivateMutex == NULL) {
         // Create the mutex that protects the linked list

cell/src/u_cell_file.c

@@ -58,6 +58,15 @@ static uCellPrivateFileListContainer_t *gpFileList = NULL;
  * STATIC FUNCTIONS
  * -------------------------------------------------------------- */
 
+/* ----------------------------------------------------------------
+ * PUBLIC FUNCTIONS: WORKAROUND FOR LINKER ISSUE
+ * -------------------------------------------------------------- */
+
+void uCellFilePrivateLink()
+{
+    //dummy
+}
+
 /* ----------------------------------------------------------------
  * PUBLIC FUNCTIONS
  * -------------------------------------------------------------- */