protobuf-c: 'inline' workaround for microsoft compilers

Per http://msdn.microsoft.com/en-us/library/z8y1yy88(v=vs.71).aspx,
Microsoft compilers do not support the Standard C keyword 'inline',
instead preferring the implementation-specific '__inline', so add a
workaround for these specific compilers.

Based on a patch from alex85k (#167).

protobuf-c/protobuf-c.c

@@ -55,6 +55,11 @@
 
 #define PROTOBUF_C__ASSERT_NOT_REACHED() assert(0)
 
+/* Workaround for Microsoft compilers. */
+#ifdef _MSC_VER
+# define inline __inline
+#endif
+
 /**
  * \defgroup internal Internal functions and macros
  *