testsuite: Import objc-dg-prune in execute.exp

The GCC-local definition of gcc-dg-prune removes extra error messages,
such as one from the linker warning about executable stacks.  This is
then used by tool-specific pruners like objc-dg-prune, defined in
objc-dg.exp.  However, objc/execute/execute.exp didn't include
objc-dg.exp, meaning that the linker warning could trigger a
failure in objc/execute/nested-func-1.m.

gcc/testsuite/
	* objc/execute/execute.exp: Load objc-dg.exp.