build-sys: add -Wimplicit-function-declaration to compiler warnings

Signed-off-by: Karel Zak <kzak@redhat.com>
swaplabel: move swapon-common to include directory

This fixes the following compiler warning.

disk-utils/swaplabel.c:175:2: warning: implicit declaration of function
'get_swap_prober' [-Wimplicit-function-declaration]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>