gbproxy: Fix build on Deiban 8

[  258s]   CC       gb_proxy.o
[  258s] gb_proxy.c: In function 'gbproxy_select_sgsn_bvc':
[  258s] gb_proxy.c:293:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
[  258s]   for (int i = 0; i < ARRAY_SIZE(cell->sgsn_bvc); i++) {
[  258s]   ^
[  258s] gb_proxy.c:293:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your
code

Change-Id: I717410b11f1ee38d49e9ca5af593cb59a244ae0a
1 file changed