Fix STATIC_ASSERT macro when using multiple .proto files.

The __COUNTER__ macro (used for generating unique names) is at least supported
by gcc, clang and Visual Studio. With this change test_compiles.c is
compilable, since no more typedefs are redefined.

Compilers/Preprocessors not supporting __COUNTER__ error's are still possible
which are hopfully handled by the usage of __LINE__ in most sittuations.

Added unit test for the problem.

tests/test_compiles.c

@@ -0,0 +1,13 @@
+/*
+ * Tests if still compile if typedefs are redfefined in STATIC_ASSERTS when
+ * proto file includes another poto file
+ */
+
+#include <stdio.h>
+#include <pb_encode.h>
+#include "callbacks2.pb.h"
+
+int main()
+{
+	return 0;
+}