metag: add missing header file
metag: add missing header file

When installing header files a warning is generated, because
the header is mandatory.

wbx-github/uclibc-ng,kraj/uclibc-ng,kraj/uclibc-ng,wbx-github/uclibc-ng,wbx-github/uclibc-ng,foss-for-synopsys-dwc-arc-processors/uClibc,brgl/uclibc-ng,kraj/uclibc-ng,brgl/uclibc-ng,brgl/uclibc-ng,foss-for-synopsys-dwc-arc-processors/uClibc,brgl/uclibc-ng,kraj/uclibc-ng,foss-for-synopsys-dwc-arc-processors/uClibc,wbx-github/uclibc-ng,foss-for-synopsys-dwc-arc-processors/uClibc