build-sys: fix find-in-path

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

Broken in commit 2b2e59e.  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>

rules.mak

@@ -138,7 +138,7 @@ endef
 # Looks in the PATH if the argument contains no slash, else only considers one
 # specific directory.  Returns an # empty string if the program doesn't exist
 # there.
-find-in-path = $(if $(find-string /, $1), \
+find-in-path = $(if $(findstring /, $1), \
         $(wildcard $1), \
         $(wildcard $(patsubst %, %/$1, $(subst :, ,$(PATH)))))