user/v9: fix -Wimplicit-function-declaration

[thesamesam]

GCC 14 and Clang 16 make this fatal:

gcc -g -O2 -Wall -I../../drbd-headers -I.. -I. -I../shared    -c -o drbdadm_main.o drbdadm_main.c
drbdadm_main.c: In function ‘main’:
drbdadm_main.c:3501:17: error: implicit declaration of function ‘include_file’ [-Wimplicit-function-declaration]
 3501 |                 include_file(f, config_test);
      |                 ^~~~~~~~~~~~