[gdb/testsuite] Check for sys/random.h in gdb.reverse/getrandom.exp

[gdb/testsuite] Check for sys/random.h in gdb.reverse/getrandom.exp

When running test-case gdb.reverse/getrandom.exp on a system with eglibc 2.19,
we run into:
...
 gdb compile failed, gdb.reverse/getrandom.c:18:24: fatal error: \
   sys/random.h: No such file or directory
  #include <sys/random.h>
                         ^
 compilation terminated.

                 === gdb Summary ===

 # of untested testcases        1
...
and:
...
UNTESTED: gdb.reverse/getrandom.exp: failed to prepare
...

Fix this by testing for the presence of the header, such that we have instead:
...
UNSUPPORTED: gdb.reverse/getrandom.exp: require failed: \
  have_system_header sys/random.h
...

Tested on x86_64-linux and i686-linux.