Mk/bsd.gecko.mk: add devel/binutils to BUILD_DEPENDS
Currently, Rust on powerpc64 elfv2 builds with GCC because of past issue with LLVM. However, this issue seems to be resolved now and I can already build Rust with Clang and then build with it other packages, like Firefox.
The problem is that, when using GCC, binutils is getting installed, but when using LLVM, it's not and there is -fno-integrated-as getting passed on ppc64.
Since it's only for the build and users building from ports have binutils installed anyway, add it for everyone.
Approved by: jbeich (maintainer)