minor bugfix

Compilation of UE softmodem failed.

targets/RT/USER/lte-uesoftmodem.c

@@ -136,7 +136,7 @@ volatile int             start_UE = 0;
 #endif
 volatile int             oai_exit = 0;
 
-static clock_source_t clock_source = internal;
+clock_source_t clock_source = internal;
 static int wait_for_sync = 0;
 
 unsigned int                    mmapped_dma=0;