clean up some prism warnings

There are a number of duplicate declarations that should be in PrismDefs.h

+// extern declarations for core functions
+extern int isExtensionSupported(const char *allExtensions, const char *extension);
+extern void extractVersionInfo(char *versionStr, int *numbers);
+extern void initializeCtxInfo(ContextInfo *ctxInfo);
+extern void initializePixelFormatInfo(PixelFormatInfo *pfInfo);
+extern void initState(ContextInfo *ctxInfo);
+extern void deletePixelFormatInfo(PixelFormatInfo *pfInfo);