extra/ios: retrieve via xcrun latest available SDK version

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>
(cherry picked from commit 040ac94d13c17933b6928201e943f0bd9a792b51)
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

extras/package/ios/build.sh

@@ -4,7 +4,7 @@ set -e
 PLATFORM=OS
 VERBOSE=no
 DEBUG=no
-SDK_VERSION=7.0
+SDK_VERSION=`xcrun --sdk iphoneos --show-sdk-version`
 SDK_MIN=6.1
 SIXTYFOURBIT_SDK_MIN=7.0
 ARCH=armv7