makeHardcodeGsettingsPatch: Fix schemaExistsFunction w/o stdbool.h

If project uses C and does not include `stdbool.h`,
it will not have available lowercase booleans.
However, since this tool targets GLib programs,
we can rely on uppercase constants defined by GLib.

pkgs/build-support/make-hardcode-gsettings-patch/hardcode-gsettings.cocci

@@ -162,4 +162,4 @@ expression SCHEMA_ID;
 identifier schema_exists_fn.fn;
 @@
 -fn(SCHEMA_ID)
-+true
++TRUE

pkgs/test/make-hardcode-gsettings-patch/fixtures/example-project-patched-with-exists-fn/main.c

@@ -74,7 +74,7 @@ void schema_id_with_path() {
 }
 
 void exists_fn_guard() {
-  if (!true) {
+  if (!TRUE) {
     return;
   }
 
@@ -89,7 +89,7 @@ void exists_fn_guard() {
 }
 
 void exists_fn_nested() {
-  if (true) {
+  if (TRUE) {
     g_autoptr(GSettings) settings = NULL;
     {
       g_autoptr(GSettingsSchemaSource) schema_source;
@@ -102,7 +102,7 @@ void exists_fn_nested() {
 }
 
 void exists_fn_unknown() {
-  if (true) {
+  if (TRUE) {
     g_autoptr(GSettings) settings = NULL;
     {
       g_autoptr(GSettingsSchemaSource) schema_source;