nouveau: Fix build, invalid extern "C" around header inclusion.

A previous patch to fix header inclusion within extern "C" neglected
to fix the occurences of this pattern in nouveau files.

When the helper to detect this issue was pushed to master, it broke
the build for the nouveau driver.  This patch fixes the nouveau build.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89477
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/tgsi/tgsi_scan.h

@@ -33,6 +33,10 @@
 #include "pipe/p_state.h"
 #include "pipe/p_shader_tokens.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * Shader summary info
  */
@@ -114,5 +118,8 @@ tgsi_scan_shader(const struct tgsi_token *tokens,
 extern boolean
 tgsi_is_passthrough_shader(const struct tgsi_token *tokens);
 
+#ifdef __cplusplus
+} // extern "C"
+#endif
 
 #endif /* TGSI_SCAN_H */

src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp

@@ -20,11 +20,9 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
-extern "C" {
 #include "tgsi/tgsi_dump.h"
 #include "tgsi/tgsi_scan.h"
 #include "tgsi/tgsi_util.h"
-}
 
 #include <set>