cosmetic: make function mncc_tx_to_gsm_cc static

The function mncc_tx_to_gsm_cc() is declared as non static but only used
from within gsm_04_08_cc.c. Lets declare it as static to increase
readability of the code

Change-Id: Icd02c669cfee6dd7e6b154e303cd0f4c148c83c4

src/libmsc/gsm_04_08_cc.c

@@ -1798,7 +1798,7 @@ static struct downstate {
 	(sizeof(downstatelist) / sizeof(struct downstate))
 
 
-int mncc_tx_to_gsm_cc(struct gsm_network *net, const union mncc_msg *msg)
+static int mncc_tx_to_gsm_cc(struct gsm_network *net, const union mncc_msg *msg)
 {
 	int i, rc = 0;
 	struct msc_a *msc_a = NULL;