Remove pbr dependency at run time
Remove pbr dependency at run time

This change is based on the following commit in the Swift tree.

  0717133 Make pbr a build-time only dependency

Change-Id: I43956f531a9928ade296236b3b605e52dc2f86f3

stackforge/swift3,tumf/swift3,swiftstack/swift3-stackforge,swiftstack/swift3-stackforge,tumf/swift3,KoreaCloudObjectStorage/swift3,stackforge/swift3,KoreaCloudObjectStorage/swift3