discover: Fix automake warnings

Change the Makfile.am relocatable output files from automake _LIBRARIES
to automake _PROGRAMS.  Also, change the output file name extension
from .o to .ro to better show these are relocatable files.

Fixes automake warnings like these:

  discover/Makefile.am: `libparser.o' is not a standard library name
  discover/Makefile.am: did you mean `libparser.a'?

Signed-off-by: Geoff Levand <geoff@infradead.org>