shellinabox: add dependency on !BR2_STATIC_LIBS

shellinabox uses <dlfcn.h>, so a !BR2_STATIC_LIBS dependency is needed.

Fixes:

  http://autobuild.buildroot.net/results/59dd88e31a5f49231c4131b840fbf1bb76e777e7/



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