GDB testsuite: Fix warnings with -std=gnu11

GDB testsuite: Fix warnings with -std=gnu11

Since upstream GCC has changed the default C language dialect to
'gnu11', it yields multiple warnings in the GDB testsuite for missing
function return types and implicit function declarations.  This patch
attempts to fix these.

gdb/testsuite/ChangeLog:

	* gdb.ada/cond_lang/foo.c (callme): Add return type.
	* gdb.base/call-sc.c (zed): Likewise.
	* gdb.base/checkpoint.c (main): Likewise.
	* gdb.base/dump.c (main): Likewise.
	* gdb.base/gcore.c (main): Likewise.
	* gdb.base/huge.c (main): Likewise.
	* gdb.base/multi-forks.c (main): Likewise.
	* gdb.base/pr10179-a.c (main): Likewise.
	* gdb.base/savedregs.c (main): Likewise.
	* gdb.base/sigaltstack.c (main): Likewise.
	* gdb.base/siginfo.c (main): Likewise.
	* gdb.base/structs.c (zed): Likewise.
	* gdb.mi/mi-stack.c (callee3, callee2, callee1, main): Likewise.
	* gdb.mi/mi-syn-frame.c (main): Likewise.
	* gdb.mi/until.c (foo, main): Likewise.
	* gdb.base/global-var-nested-by-dso.c (b_main, c_main): Declare.
	* gdb.base/solib-weak.c (foo): Declare.
	* gdb.base/attach-twice.c: Include stdio.h.
	* gdb.base/weaklib1.c: Likewise.
	* gdb.base/weaklib2.c: Likewise.
	* gdb.base/catch-signal-fork.c: Include stdio.h and sys/wait.h.
	* gdb.mi/mi-condbreak-call-thr-state-mt.c: Include stdio.h and
	unistd.h.
	* gdb.base/attach-pie-misread.c: Include stdlib.h.
	* gdb.mi/mi-exit-code.c: Likewise.
	* gdb.base/break-interp-lib.c: Include string.h.
	* gdb.base/coremaker.c: Likewise.
	* gdb.base/testenv.c: Likewise.
	* gdb.python/py-finish-breakpoint.c: Likewise.
	* gdb.base/inferior-died.c: Include sys/wait.h.
	* gdb.base/fileio.c: Include time.h.
	* gdb.base/async-shell.c: Include unistd.h.
	* gdb.base/dprintf-non-stop.c: Likewise.
	* gdb.base/info-os.c: Likewise.
	* gdb.mi/mi-console.c: Likewise.
	* gdb.mi/watch-nonstop.c: Likewise.
	* gdb.python/py-events.c: Likewise.
	* gdb.base/async.c (baz): Move up before its invocation.
	* gdb.base/code_elim2.c (my_global_func): Likewise.
	* gdb.base/skip-solib-lib.c (multiply): Likewise.
	* gdb.base/advance.c (func2): Likewise.