Fix "enumeration values not handled in switch" error in testsuite

Fix "enumeration values not handled in switch" error in testsuite

When running the testsuite with clang, gdb.base/sigaltstack.c
fails to compile with the following error:
  warning: enumeration values 'LEAF' and 'NR_LEVELS' not handled
    in switch [-Wswitch]

This prevents the gdb.base/sigaltstack.exp from executing.
This commit fixes.

gdb/testsuite/ChangeLog:

	* gdb.base/sigaltstack.c (catcher): Add default case to switch
	statement.