icu: fix parallel builds

The icu build system seems to have a race condition, which gets triggered
by high BR2_JLEVEL settings, so disable parallel builds.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/icu/icu.mk

@@ -12,6 +12,7 @@ ICU_INSTALL_STAGING = YES
 ICU_CONF_OPT = --with-cross-build=$(HOST_ICU_DIR)/source --disable-samples \
 		--disable-tests
 HOST_ICU_CONF_OPT = --disable-samples --disable-tests
+ICU_MAKE = $(MAKE1)
 ICU_SUBDIR = source
 HOST_ICU_SUBDIR = source