util: fix unknown pragma warning on msvc

MSVC has no idea about these pragmas, and spews warnings about them,
making it hard to spot real problems. So let's only use these macros on
GCC.

Fixes: 2ec290cd92 ("util: Fix/silence variable shadowing warnings")
Reviewed-by: Tony Wasserka <tony.wasserka@gmx.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7633>

src/util/format/u_format.h

@@ -383,8 +383,10 @@ typedef void (*util_format_fetch_rgba_func_ptr)(void *dst, const uint8_t *src,
                                                 unsigned i, unsigned j);
 
 /* Silence warnings triggered by sharing function/struct names */
+#ifdef __GNUC__
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wshadow"
+#endif
 const struct util_format_description *
 util_format_description(enum pipe_format format) ATTRIBUTE_CONST;
 
@@ -393,7 +395,9 @@ util_format_pack_description(enum pipe_format format) ATTRIBUTE_CONST;
 
 const struct util_format_unpack_description *
 util_format_unpack_description(enum pipe_format format) ATTRIBUTE_CONST;
+#ifdef __GNUC__
 #pragma GCC diagnostic pop
+#endif
 
 /**
  * Returns a function to fetch a single pixel (i, j) from a block.