Fix typo in _initialize_alpha_linux_nat prototype

	* alpha-linux-nat.c (_initialize_alpha_linux_nat): Fix prototype.
Add return value for non-void function return statements to fix error in clang build.

Clang defaults this warning to an error, breaking the build & causing
these tests not to run.

gdb/testsuite/

	* gdb.mi/non-stop.c: Add return value for non-void function return
	statement.
	* gdb.threads/staticthreads.c: Ditto.