srcfiles: Fix --enable-gcov (BUILD_STATIC) build

srcfiles: Fix --enable-gcov (BUILD_STATIC) build

When configuring with --enable-gcov we build most things static.
Including libdebuginfod. The src Makefile was only setup for a
shared library build of libdebuginfod.so. Fix this by providing
a static libdebuginfod in case of BUILD_STATIC.

This fixes the builder.sourceware.org elfutils-snapshots-coverage
and provides fresh coverage reports again at
https://snapshots.sourceware.org/elfutils/coverage/latest/

	* Makefile.am (BUILD_STATIC): Provide libdebuginfod.a

Signed-off-by: Mark Wielaard <mark@klomp.org>