script/autobuild.py: let nm_grep_symbols ignore __gcov_ symbols

script/autobuild.py: let nm_grep_symbols ignore __gcov_ symbols

Currently the gcov build currently fails with the following error:

samba-libs: [allshared-no-public-nss_winbind] Running nm ./bin/plugins/libnss_winbind.so.2 | egrep -v ' (__bss_start|_edata|_init|_fini|_end)' | egrep -v ' T _nss_winbind_' |egrep ' [BDGTRVWS] ' && exit 1; exit 0; in '/tmp/samba-testbase/samba-libs/.'
0000000000232458 B __gcov_error_file
0000000000226340 D __gcov_master
000000000001c080 T __gcov_sort_n_vals
00000000002324a0 B __gcov_var
samba-libs: [allshared-no-public-nss_winbind] failed 'nm ./bin/plugins/libnss_winbind.so.2 | egrep -v ' (__bss_start|_edata|_init|_fini|_end)' | egrep -v ' T _nss_winbind_' |egrep ' [BDGTRVWS] ' && exit 1; exit 0;' with status 1

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Sun Feb  6 13:39:09 UTC 2022 on sn-devel-184