[gdb/testsuite] Add missing include in gdb.base/rtld-step.exp
On fedora rawhide, with test-case gdb.base/rtld-step.exp I get:
...
static-pie-static-libc.c: In function '_start':^M
static-pie-static-libc.c:1:22: error: \
implicit declaration of function '_exit' [-Wimplicit-function-declaration]^M
1 | void _start (void) { _exit (0); }^M
| ^~~~~^M
compiler exited with status 1
...
UNTESTED: gdb.base/rtld-step.exp: failed to compile \
(-static-pie not supported or static libc missing)
...
Fix this by adding the missing include.
Tested on aarch64-linux.
Approved-by: Kevin Buettner <kevinb@redhat.com>