drivers: rmnet_perf: Fix function declaration warning

Fixes the following warning/error with clang 15:

../techpack/data/drivers/rmnet/perf/rmnet_perf_opt.c:715:27: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
rmnet_perf_free_hash_table()
                          ^
                           void
1 warning and 1 error generated.

Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Change-Id: I5822104dfbd4514d914f33484fa25d312b565bda