drat-trim: fix `gcc-14` build
Without the change the build on `gcc-14` fails as:
drat-trim.c: In function 'read_lit':
drat-trim.c:989:10: error: implicit declaration of function 'getc_unlocked' [-Wimplicit-function-declaration]
989 | lc = getc_unlocked (S->proofFile);
| ^~~~~~~~~~~~