complibs/mpc: fix MPC 0.8.1 build with MPFR 3.0.0
This is the change introduced by revision 734 of MPC repository.
Author: Paul Zimmermann <Paul.Zimmermann@loria.fr>
Revision log: [acos.c] fixed problem with GMP_RNDA (should be MPFR_RNDA, and code was wrong)
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>