[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.
missing-header in 1 minute and 7 seconds3 jobs for