Add a missing extern keyword that prevented compilation on OS X.

This commit was SVN r13853.

ompi/mca/mtl/base/base.h

@@ -33,7 +33,7 @@
 extern "C" {
 #endif
 
-OMPI_DECLSPEC mca_mtl_base_component_t* ompi_mtl_base_selected_component;
+OMPI_DECLSPEC extern mca_mtl_base_component_t* ompi_mtl_base_selected_component;
     
 OMPI_DECLSPEC int ompi_mtl_base_open(void);
 OMPI_DECLSPEC int ompi_mtl_base_select(bool enable_progress_threads,