Avoid compiling xcassets in some cases

Added a condition to avoid compiling xcassets when WRAPPER_EXTENSION
is undefined, as it would be in the case of static libraries. This prevents
trying to copy the compiled files to a directory that does not exist.

lib/cocoapods/generator/copy_resources_script.rb

@@ -137,7 +137,7 @@ EOS
 
       XCASSETS_COMPILE = <<EOS
 
-if [ `find . -name '*.xcassets' | wc -l` -ne 0 ]
+if [[ -n "${WRAPPER_EXTENSION}" ]] && [ `find . -name '*.xcassets' | wc -l` -ne 0 ]
 then
   DEVICE=`if [ "${TARGETED_DEVICE_FAMILY}" -eq 1 ]; then echo "iphone"; else echo "ipad"; fi`
   find "${PWD}" -name "*.xcassets" -print0 | xargs -0 actool --output-format human-readable-text --notices --warnings --platform "${PLATFORM_NAME}" --minimum-deployment-target "${IPHONEOS_DEPLOYMENT_TARGET}" --target-device "${DEVICE}" --compress-pngs --compile "${BUILT_PRODUCTS_DIR}/${PRODUCT_NAME}.${WRAPPER_EXTENSION}"