Introduce the PACMAN_EXTRAPACKAGES variable in PKGBUILD to allow users
to specify which additional packages are built by the pacman-pkg target.

Previously, the api-headers package was always included, and the headers
package was included only if CONFIG_MODULES=y. With this change, both
headers and api-headers packages are included by default. Users can now
control this behavior by setting PACMAN_EXTRAPACKAGES to a
space-separated list of desired extra packages or leaving it empty to
exclude all.

For example, to build only the base package without extras:

make pacman-pkg PACMAN_EXTRAPACKAGES=""
Signed-off-by: Jose Fernandez <jose.fernandez@linux.dev>
Reviewed-by: Peter Jung <ptr1337@cachyos.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Christian Heusel <christian@heusel.eu>
Tested-by: Christian Heusel <christian@heusel.eu>
Acked-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>