global: fix compile error at pragma GCC diagnostic

* src/global.c (_gcry_vcontrol): Move "pragma GCC diagnostics" outside
function.

Building with GCC-4.2 (Ubuntu 8.04) failed with error:
global.c: In function '_gcry_vcontrol':
global.c:714: error: #pragma GCC diagnostic not allowed inside functions
global.c:715: error: #pragma GCC diagnostic not allowed inside functions
global.c:742: error: #pragma GCC diagnostic not allowed inside functions

• GnuPG-bug-id: T5264
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>