Fix undeclared function rb_mmtk_enabled_p when compiling

I noticed that in CI and locally the `mmtk` branch wasn't compiling and
failing on the following compilation errors:

```
gc.c:10074:9: error: call to undeclared function 'rb_mmtk_enabled_p'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    if (rb_mmtk_enabled_p()) {
        ^
```

```
error.c:1076:39: error: call to undeclared function 'rb_mmtk_enabled_p'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    if (rb_current_execution_context(!rb_mmtk_enabled_p())) {
                                      ^
```

The `internal/mmtk_support.h` header is included fine, but these two
lines were using `ifdef USE_MMTK` rather than `if USE_MMTK`. Changing
these allows CRuby mmtk branch to compile correctly.

error.c

@@ -1070,7 +1070,7 @@ rb_bug_for_fatal_signal(ruby_sighandler_t default_sighandler, int sig, const voi
  const char *file = NULL;
  int line = 0;
 
-#ifdef USE_MMTK
+#if USE_MMTK
  // When using MMTk, this function may be called from GC worker threads,
  // in which case there will not be a Ruby execution context.
  if (rb_current_execution_context(!rb_mmtk_enabled_p())) {

gc.c

@@ -10070,7 +10070,7 @@ rb_gc_prepare_heap(void)
 static int
 gc_is_moveable_obj(rb_objspace_t *objspace, VALUE obj)
 {
-#ifdef USE_MMTK
+#if USE_MMTK
  if (rb_mmtk_enabled_p()) {
  rb_bug("Function %s should not be called when MMTk is enabled.", RUBY_FUNCTION_NAME_STRING);
  }