rsl.c: Fix compiler error on gcc-9.2.1
rsl.c: Fix compiler error on gcc-9.2.1

rsl.c: In function ‘rsl_rx_ipac_XXcx’:
rsl.c:2147:39: error: ‘%s’ directive output may be truncated writing up to 255 bytes into a region of size 28 [-Werror=format-truncation=]
 2147 |   snprintf(cname, sizeof(cname), "bts@%s", ipstr);
      |                                       ^~
rsl.c:2147:3: note: ‘snprintf’ output between 5 and 260 bytes into a destination of size 32
 2147 |   snprintf(cname, sizeof(cname), "bts@%s", ipstr);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: Id982a814f401e304327d25c77666f039bc156c1f

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts