doc: fix troff warning in libgrokj2k.3 man page

Fix the following warning in libgrokj2k.3 man page:

-----------------------------------------------------------------------
$ LC_ALL=it_IT.UTF-8 MANROFFSEQ='' MANWIDTH=80 \
  man --warnings -E UTF-8 -l -Tutf8 -Z doc/man/man3/libgrokj2k.3 >/dev/null
troff: <standard input>:127: warning: macro 'p' not defined
-----------------------------------------------------------------------

The issue was pointed out by the 'lintian' tool from Debian:

W: libgrokj2k1-dev: manpage-has-errors-from-man usr/share/man/man3/libgrokj2k.3.gz 127: warning: macro 'p' not defined

Split executable modules from the library object, in particular this is
to prevent the exit() symbol to become part of the library, as this may
raise some flags in linting tools like "lintian" from Debian, e.g.:

X: libgrokj2k1: shlib-calls-exit usr/lib/x86_64-linux-gnu/libgrokj2k.so.7.1.0

Still keep executable sources grouped in a new GROK_EXECUTABLES_SRCS
variable to be able to run cppcheck on all of them.