deps: backport GYP fix to fix AIX shared suffix

Required to support the shared library builds on AIX - this sets the
shared library suffix within GYP to .a instead of .so on AIX
My patch: https://codereview.chromium.org/2492233002/ was landed as
as part of this one which fixed some other (not required, but
included for completeness of the backport) changes:

Ref: https://codereview.chromium.org/2511733005/