Remove milenage library from linking setting (thanks Tzafrir Cohen for the patch)

[nanang]

Currently milenage will always be included in the linking setting, while it is used only when PJSIP_HAS_DIGEST_AKA_AUTH is set (to enable IMS/3GPP digest AKA authentication), which is considered very rare, and extra -l flags have their toll with dynamic linking.

So by removing it from linking setting, any application that enables AKA authentication would need to link with milenage library from third_party directory.

Note that the milenage library should be built manually, i.e: by typing 'make' from directory third_party/build/milenage, and the milenage library could be found in third_party/lib.

[nanang]

In 4720:

Close #1728: Removed milenage library from linking setting.