tests/Makefile: Only run vhost-user-test on Linux

vhost-user-test uses the linux/vhost.h header, so it must only be
enabled if CONFIG_LINUX is defined. (Previously it was enabled
for CONFIG_POSIX, which broke 'make check' on MacOSX.)
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

tests/Makefile

@@ -158,7 +158,7 @@ gcov-files-i386-y += hw/usb/hcd-ehci.c
 gcov-files-i386-y += hw/usb/hcd-uhci.c
 gcov-files-i386-y += hw/usb/dev-hid.c
 gcov-files-i386-y += hw/usb/dev-storage.c
-check-qtest-i386-$(CONFIG_POSIX) += tests/vhost-user-test$(EXESUF)
+check-qtest-i386-$(CONFIG_LINUX) += tests/vhost-user-test$(EXESUF)
 check-qtest-x86_64-y = $(check-qtest-i386-y)
 gcov-files-i386-y += i386-softmmu/hw/timer/mc146818rtc.c
 gcov-files-x86_64-y = $(subst i386-softmmu/,x86_64-softmmu/,$(gcov-files-i386-y))