test: Add prototype for Xmureallocarray to silence warning
In file included from reallocarray.c:50:
../src/reallocarray.c:35:1: warning: no previous prototype for ‘Xmureallocarray’ [-Wmissing-prototypes]
35 | Xmureallocarray(void *optr, size_t nmemb, size_t size)
| ^~~~~~~~~~~~~~~