externals: Update Glad to latest version used by Citra.

externals/glad/Readme.md

 These files were generated by the [glad](https://github.com/Dav1dde/glad) OpenGL loader generator and have been checked in as-is. You can re-generate them using glad with the following command:
 
 ```
-python -m glad --profile core --out-path glad/ --api gl=3.3,gles=3.0
+python -m glad --profile core --out-path glad/ --api gl=3.3 --generator=c
 ```

externals/glad/include/KHR/khrplatform.h

@@ -26,7 +26,7 @@
 
 /* Khronos platform-specific types and definitions.
  *
- * $Revision: 23298 $ on $Date: 2013-09-30 17:07:13 -0700 (Mon, 30 Sep 2013) $
+ * $Revision: 32517 $ on $Date: 2016-03-11 02:41:19 -0800 (Fri, 11 Mar 2016) $
  *
  * Adopters may modify this file to suit their platform. Adopters are
  * encouraged to submit platform specific modifications to the Khronos
@@ -101,6 +101,8 @@
 #   define KHRONOS_APICALL __declspec(dllimport)
 #elif defined (__SYMBIAN32__)
 #   define KHRONOS_APICALL IMPORT_C
+#elif defined(__ANDROID__)
+#   define KHRONOS_APICALL __attribute__((visibility("default")))
 #else
 #   define KHRONOS_APICALL
 #endif
@@ -223,7 +225,7 @@ typedef signed   short int     khronos_int16_t;
 typedef unsigned short int     khronos_uint16_t;
 
 /*
- * Types that differ between LLP64 and LP64 architectures - in LLP64, 
+ * Types that differ between LLP64 and LP64 architectures - in LLP64,
  * pointers are 64 bits, but 'long' is still 32 bits. Win64 appears
  * to be the only LLP64 architecture in current use.
  */