package/git: fix build with curl

curl-config path can be set through ac_cv_prog_CURL_CONFIG and not
ac_cv_prog_curl_config

Fixes:
 - http://autobuild.buildroot.org/results/4b22f761795d8760dac6ddfd40934259f0135a4d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/git/git.mk

@@ -35,7 +35,7 @@ ifeq ($(BR2_PACKAGE_LIBCURL),y)
 GIT_DEPENDENCIES += libcurl
 GIT_CONF_OPTS += --with-curl
 GIT_CONF_ENV += \
-	ac_cv_prog_curl_config=$(STAGING_DIR)/usr/bin/$(LIBCURL_CONFIG_SCRIPTS)
+	ac_cv_prog_CURL_CONFIG=$(STAGING_DIR)/usr/bin/$(LIBCURL_CONFIG_SCRIPTS)
 else
 GIT_CONF_OPTS += --without-curl
 endif