lib/zstd.h: move pragma before static

otherwise will cause dev-python/zstandard build failed when compiling with
clang as reported at https://bugs.gentoo.org/950259

the root cause is pycparser, which is unfixed since reported 2.5 years
ago, :(

Signed-off-by: Z. Liu <zhixu.liu@gmail.com>

lib/zstd.h

@@ -1868,15 +1868,14 @@ ZSTDLIB_STATIC_API const ZSTD_DDict* ZSTD_initStaticDDict(
 typedef void* (*ZSTD_allocFunction) (void* opaque, size_t size);
 typedef void  (*ZSTD_freeFunction) (void* opaque, void* address);
 typedef struct { ZSTD_allocFunction customAlloc; ZSTD_freeFunction customFree; void* opaque; } ZSTD_customMem;
-static
-#ifdef __GNUC__
-__attribute__((__unused__))
-#endif
-
 #if defined(__clang__) && __clang_major__ >= 5
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wzero-as-null-pointer-constant"
 #endif
+static
+#ifdef __GNUC__
+__attribute__((__unused__))
+#endif
 ZSTD_customMem const ZSTD_defaultCMem = { NULL, NULL, NULL };  /**< this constant defers to stdlib's functions */
 #if defined(__clang__) && __clang_major__ >= 5
 #pragma clang diagnostic pop