meson: require nm again on Unix systems

This was made optional in ff9bf223c2 ("meson: make nm binary optional")
for Windows, but proper windows has been added and `nm` is now only used
on Unix systems.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviwed-by: Dylan Baker <dylan@pnwbakers>

meson.build

@@ -1685,8 +1685,8 @@ if host_machine.system() == 'windows'
   with_symbols_check = prog_dumpbin.found() and with_tests
   symbols_check_args = ['--dumpbin', prog_dumpbin.path()]
 else
-  prog_nm = find_program('nm', required : false)
-  with_symbols_check = prog_nm.found() and with_tests
+  prog_nm = find_program('nm')
+  with_symbols_check = with_tests
   symbols_check_args = ['--nm', prog_nm.path()]
 endif