xen/arm: unbreak arm64 build for older toolchains

xen/arm: unbreak arm64 build for older toolchains

Commit 4941bfb "xen/arm64: macros: Introduce an assembly macro to alias
x30" moved

  lr      .req    x30

to macros.h. A later patch (1396dab "xen/arm64: head: Don't clobber
x30/lr in the macro PRINT") started to use "lr" in head.S, however, it
didn't add an #include macros.h to head.S. This commit fixes it.

The lack of alias breaks the build with
gcc-linaro-5.3.1-2016.05-x86_64_aarch64-linux-gnu. The alias was added
later to binutils 2.29 in 2017.

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>