gallium/tgsi_from_mesa: Add 'extern "C"' to be able to include from C++

The r600/nir backend is in C++ and needs to include this file.

Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3286>

src/gallium/auxiliary/tgsi/tgsi_from_mesa.h

@@ -32,6 +32,11 @@
 
 #include "compiler/shader_enums.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
 void
 tgsi_get_gl_varying_semantic(gl_varying_slot attr,
                              bool needs_texcoord_semantic,
@@ -84,4 +89,8 @@ tgsi_processor_to_shader_stage(unsigned processor)
    }
 }
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* TGSI_FROM_MESA_H */