misc: Move the bsc_parse_reg to libcommom and name it gsm_parse_reg

Move the regexp parsing code from the NAT to libcommon as it will
be used by the NAT and BSC code. This also adds the #include <regex.h>
include to gsm_data. This header should be split up.
diff --git a/openbsc/tests/bsc-nat/bsc_nat_test.c b/openbsc/tests/bsc-nat/bsc_nat_test.c
index d198424..d6105ad 100644
--- a/openbsc/tests/bsc-nat/bsc_nat_test.c
+++ b/openbsc/tests/bsc-nat/bsc_nat_test.c
@@ -745,15 +745,15 @@
 		nat_lst = bsc_nat_acc_lst_get(nat, "nat");
 		bsc_lst = bsc_nat_acc_lst_get(nat, "bsc");
 
-		if (bsc_parse_reg(nat_entry, &nat_entry->imsi_deny_re, &nat_entry->imsi_deny,
+		if (gsm_parse_reg(nat_entry, &nat_entry->imsi_deny_re, &nat_entry->imsi_deny,
 			      cr_filter[i].nat_imsi_deny ? 1 : 0,
 			      &cr_filter[i].nat_imsi_deny) != 0)
 			abort();
-		if (bsc_parse_reg(bsc_entry, &bsc_entry->imsi_allow_re, &bsc_entry->imsi_allow,
+		if (gsm_parse_reg(bsc_entry, &bsc_entry->imsi_allow_re, &bsc_entry->imsi_allow,
 			      cr_filter[i].bsc_imsi_allow ? 1 : 0,
 			      &cr_filter[i].bsc_imsi_allow) != 0)
 			abort();
-		if (bsc_parse_reg(bsc_entry, &bsc_entry->imsi_deny_re, &bsc_entry->imsi_deny,
+		if (gsm_parse_reg(bsc_entry, &bsc_entry->imsi_deny_re, &bsc_entry->imsi_deny,
 			      cr_filter[i].bsc_imsi_deny ? 1 : 0,
 			      &cr_filter[i].bsc_imsi_deny) != 0)
 			abort();