Fix small bug in compile.exp

compile.exp generally does not work for me on Fedora 38.  However, I
sent a GCC patch to fix the plugin crash.  With that patch, I get this
error from one test in compile.exp:

gdb command line:1:22: warning: initialization of 'int (*)(int)' from incompatible pointer type 'int (*)()' [-Wincompatible-pointer-types]

This patch adds a cast to compile.exp.  This makes the test pass.

Reviewed-by: Keith Seitz <keiths@redhat.com>