Add math/gmp to LIB_DEPENDS

[jrm]

Summary

Add math/gmp to LIB_DEPENDS

Q/A:
Error: /usr/local/bin/certtool is linked to /usr/local/lib/libgmp.so.10 from
math/gmp but it is not declared as a dependency
Warning: you need LIB_DEPENDS+=libgmp.so:math/gmp

Test Plan

portlint -C: Ok
testport: OK (poudriere: 11.0-RC3,amd64,default options)

[swills]

Approved