[fix] add dev headers to _SOURCES

Track development headers in src/Makefile.am _SOURCES to make sure that
`make dist` will include them in the distribution.

Might also fix some errors when modifying a header and recompiling.

See https://www.gnu.org/software/automake/manual/html_node/Headers.html
for the guidelines.
3 jobs for missing-header in 1 minute and 7 seconds
Status Job ID Name Coverage
  Quality
passed #76533
integration
repoquality

00:00:03

 
  Build
passed #76534
integration
excit

00:01:03

passed #76535
integration
excit-check

00:00:24