[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 <[email protected]>

gdb/testsuite/gdb.base/rtld-step.exp

@@ -86,7 +86,10 @@ set rtld_flags [list debug additional_flags=[list -static-pie -fPIE \
 						  -nostdlib -static -lc]]
 
 if { ![gdb_can_simple_compile static-pie-static-libc \
-	   "void _start (void) { _exit (0); }" \
+	   {
+	       #include <unistd.h>
+	       void _start (void) { _exit (0); }
+	   } \
 	   executable $rtld_flags] } {
     set reason "-static-pie not supported or static libc missing"
     untested "failed to compile ($reason)"