Get rid of the header guards in shaders_common.h/glsl_common.h

gfx/drivers/gl_shaders/glsl/frag/shaders_common.h

-#ifndef _SHADERS_COMMON
-#define _SHADERS_COMMON
-
+#undef VERTEX
+#define FRAGMENT
 #include "../glsl_common.h"
-
-#endif

gfx/drivers/gl_shaders/glsl/glsl_common.h

-#ifndef _SHADERS_COMMON
-#define _SHADERS_COMMON
+#undef GLSL_DERIV_PREAMBLE
+#undef GLSL_PREAMBLE
+#undef GLSL
+#undef GLSL_300
+#undef GLSL_330
 
 #define GLSL_DERIV_PREAMBLE() "#extension GL_OES_standard_derivatives : enable\n"
 #define GLSL_PREAMBLE() \
@@ -21,5 +24,3 @@
 #define GLSL_300(src)   "#version 300 es\n" GLSL_PREAMBLE() #src
 #define GLSL_330(src)   "#version 330 core\n" GLSL_PREAMBLE() #src
 #endif
-
-#endif

gfx/drivers/gl_shaders/glsl/vert/shaders_common.h

-#ifndef _SHADERS_COMMON
-#define _SHADERS_COMMON
-
+#undef FRAGMENT
+#undef VERTEX
+#define VERTEX
 #include "../glsl_common.h"
-
-#endif