package/gauche: fix build error on BR2_STATIC_LIBS

Building gauche depends on dynamic link, so add dependency on
!BR2_STATIC_LIBS.

Fix:
http://autobuild.buildroot.org/results/da5/da5b9605552d4914c5e6f0d890367b92536419c1



Signed-off-by: Hiroshi Kawashima <kei-k@ca2.so-net.ne.jp>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>