Fails to build on FreeBSD due to missing include

Regressed by f769e464. Workaround: -Dwith_secctx=disabled

$ meson _build
$ meson compile -C _build
[...]
../src/secctx.c:19:6: error: call to undeclared library function 'strcmp' with type 'int (const char *, const char *)'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
   19 |         if (strcmp(interface, "wp_security_context_manager_v1") == 0) {
      |             ^
../src/secctx.c:19:6: note: include the header <string.h> or explicitly provide a declaration for 'strcmp'
../src/secctx.c:90:2: error: call to undeclared library function 'strncpy' with type 'char *(char *, const char *, unsigned long)'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
   90 |         strncpy(sockaddr.sun_path, sock_path, sizeof(sockaddr.sun_path) - 1);
      |         ^
../src/secctx.c:90:2: note: include the header <string.h> or explicitly provide a declaration for 'strncpy'
2 errors generated.