Setting PATH so that the 'plistbuddy' utility can be found, in a typical OS X environment

Restoring a line from the older version (GYP-days) of the
build_ios_libs.sh script: modifying PATH so that the
PlistBuddy utility can be successfully found and called,
as it normally is not available under the PATH directories
in a typical OS X environment (even on developer configs).

NOTRY=True
BUG=webrtc:6372

Review-Url: https://codereview.webrtc.org/2463623002
Cr-Commit-Position: refs/heads/master@{#14834}

webrtc/build/ios/build_ios_libs.sh

@@ -13,6 +13,9 @@
 # Exit on errors.
 set -e
 
+# Environment
+export PATH=/usr/libexec:$PATH
+
 SCRIPT_DIR=$(cd $(dirname $0) && pwd)
 WEBRTC_BASE_DIR=${SCRIPT_DIR}/../../..