[build] Add missing header in src/Makefile.am.
[build] Add missing header in src/Makefile.am.

Makes 'make distcheck' pass.

janmarthedal/mathml2svg,wavewave/lasem,gjtorikian/lasem,gjtorikian/lasem,gjtorikian/lasem,janmarthedal/mathml2svg,wavewave/lasem,janmarthedal/mathml2svg,wavewave/lasem,janmarthedal/mathml2svg,gjtorikian/lasem,wavewave/lasem