Skip compiling C-included files during the build (#1386)

* Update the makefile to skip .inc.c files when building

* Rename all C-included source files to .inc.c

src/libultra/gu/sins.c

@@ -1,5 +1,6 @@
 #include "ultra64.h"
-#include "sintable.c"
+
+#include "sintable.inc.c"
 
 s16 sins(u16 x) {
     s16 value;