Fix a compilation error when compiling with khrplatform.h.

source/glbind_template.h

@@ -23,6 +23,7 @@ extern "C" {
 The official OpenGL headers have a dependency on a header called khrplatform.h. From what I can see it's mainly just for sized types. Since glbind is a
 single header, and that we can't just copy-and-paste the contents of khrplatform.h due to licensing, we need to do our own sized type declarations.
 */
+#ifndef __khrplatform_h_
 #include <stddef.h> /* For size_t. */
 #ifdef _MSC_VER
     #if defined(__clang__)
@@ -92,6 +93,7 @@ single header, and that we can't just copy-and-paste the contents of khrplatform
     #endif
 #endif
 typedef float khronos_float_t;
+#endif  /* __khrplatform_h_ */
 
 /* Platform headers. */
 #if defined(GLBIND_WGL)