gallium/hud: move signo declaration inside PIPE_OS_UNIX block
gallium/hud: move signo declaration inside PIPE_OS_UNIX block

To silence unused var warning with MSVC, MinGW.

Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler