aircrack-ng: don't use -ldl in a static lib configuration

aircrack-ng: don't use -ldl in a static lib configuration

This commit gets rid of the -ldl flag used when
BR2_PREFER_STATIC_LIB=y. First because it does not make sense: when
you use a pure static library configuration, you don't have shared
library support, so you don't have libdl.

The occurences of -ldl were added in commit
1141d99aae9e1b139509ad71c969e6705417a48b ("aircrack-ng: fix static
build") and commit bed1490d0f58dc963d2144f2fe6b705f9f36d848
("aircrack-ng: fix statically linked build"), at a time were sqlite
indeed was linked against libdl even in static library
configurations. But this is no longer the case since
bd56cd6b4cc75511ffacfe020a8de3c1aad44fc9 ("sqlite: Disable dynamic
extention if static library is seleted").

This means we can now get rid of the -ldl flag when building
aircrack-ng, which means it can build properly in a pure static
library configuration.

Fixes:

  http://autobuild.buildroot.org/results/bfd/bfd1ca4afdd32e3cc251aab7ee2ae5e5293ced1c/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>