Replace GLFWuint64 with uint64_t

C99 stdint.h is provided by VS 2010 and later.  GLFW has not provided
testing or binaries for VS 2008 for several releases.

For earlier versions of VS there are third-party alternatives:

https://msinttypes.googlecode.com/svn/trunk/stdint.h
http://www.azillionmonkeys.com/qed/pstdint.h

This change does not affect the ABI.

src/win32_platform.h

@@ -275,7 +275,7 @@ typedef struct _GLFWcursorWin32
 typedef struct _GLFWtimeWin32
 {
     GLFWbool            hasPC;
-    GLFWuint64          frequency;
+    uint64_t            frequency;
 
 } _GLFWtimeWin32;