ngrep: fix static build

Commit 54eeb08d6498e54cc77e212e4f9873b579f76638 wrongly replaced
`$(STAGING_DIR)/usr/bin/pcap-config --static --additional-libs` by
"$(STAGING_DIR)/usr/bin/pcap-config --static --additional-libs"

Fixes:
 - http://autobuild.buildroot.org/results/3dd8c768beef075494d6be7eff4cbb57b5ec8a41

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ngrep/ngrep.mk

@@ -13,7 +13,7 @@ NGREP_INSTALL_STAGING = YES
 NGREP_AUTORECONF = YES
 
 ifeq ($(BR2_STATIC_LIBS),y)
-NGREP_CONF_ENV += LIBS="$(STAGING_DIR)/usr/bin/pcap-config --static --additional-libs"
+NGREP_CONF_ENV += LIBS=`$(STAGING_DIR)/usr/bin/pcap-config --static --additional-libs`
 endif
 
 NGREP_CONF_OPTS = \