C conformance: Include stdbool.h to define `bool`

`bool` is otherwise not defined in C.

modules/gdnative/include/gdnative/math_defs.h

@@ -35,6 +35,7 @@
 extern "C" {
 #endif
 
+#include <stdbool.h>
 #include <stdint.h>
 
 ////// bool