linux-capture: Fix Ubuntu 21.10 builds

Ubuntu 21.10 provides PipeWire 0.3.32 which is missing the
`SPA_POD_PROP_FLAG_DONT_FIXATE` required for proper DMA-BUF negotiation.
Since this isn't implemented in the DE's of this Ubuntu version just
defining this flag won't have any impact.

Revert after support for Ubuntu 21.10 ended.

plugins/linux-capture/CMakeLists.txt

@@ -46,7 +46,7 @@ set(linux-capture_LIBRARIES
 
 option(ENABLE_PIPEWIRE "Enable PipeWire support" ON)
 if(ENABLE_PIPEWIRE)
-	find_package(PipeWire 0.3.33 QUIET)
+	find_package(PipeWire 0.3.32 QUIET)
 	find_package(Libdrm QUIET) # we require libdrm/drm_fourcc.h to build
 	find_package(Gio QUIET)
 

plugins/linux-capture/pipewire.c

@@ -38,6 +38,10 @@
 #include <spa/param/video/type-info.h>
 #include <spa/utils/result.h>
 
+#ifndef SPA_POD_PROP_FLAG_DONT_FIXATE
+#define SPA_POD_PROP_FLAG_DONT_FIXATE (1 << 4)
+#endif
+
 #define REQUEST_PATH "/org/freedesktop/portal/desktop/request/%s/obs%u"
 #define SESSION_PATH "/org/freedesktop/portal/desktop/session/%s/obs%u"