package/mongoose: fix build with gcc 4.8

Fix the following build failure with gcc 4.8 raised since bump to
version 7.8 in commit f9e51e11:

/home/buildroot/autobuild/run/instance-2/output-1/build/mongoose-7.8/mongoose.c: In function 'mg_iotest':
/home/buildroot/autobuild/run/instance-2/output-1/build/mongoose-7.8/mongoose.c:4484:3: error: 'for' loop initial declarations are only allowed in C99 mode
   for (struct mg_connection *c = mgr->conns; c != NULL; c = c->next) {
   ^

Fixes:
 - http://autobuild.buildroot.org/results/c538c6d8e87e4e24097c9878ee15083802276505



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>