Fix spelling, the GNU make text functions is not called "find-string"
but "findstring".

Broken in commit 2b2e59e6.  Fairly harmless: its only use is in
tests/tcg/Makefile, where the bug can cause the I386_TESTS not to
run when they should.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>