Fix build warnings

[martinpitt]

Just saw this on my machine, and it breaks tools/make-sysext:

src/ws/cockpithandlers.c: In function ‘build_environment’:
src/ws/cockpithandlers.c:351:8: error: unused variable ‘i’ [-Werror=unused-variable]
  351 |   gint i;
      |        ^
src/ws/cockpithandlers.c:350:15: error: unused variable ‘osr’ [-Werror=unused-variable]
  350 |   JsonObject *osr;
      |               ^~~
src/ws/cockpithandlers.c:348:16: error: unused variable ‘value’ [-Werror=unused-variable]
  348 |   const gchar *value;
      |                ^~~~~
cc1: all warnings being treated as errors
  CC       src/common/libcockpit_common_a-cockpittemplate.o
make[1]: *** [Makefile:3353: src/ws/libcockpit_ws_a-cockpithandlers.o] Error 1

[allisonkarlitskaya]

thanks!