package/bullet: needs dynamic library

Build without dlfcn.h fails because bullet3 is not disabled since
commit 5f154799

Fixes:
 - http://autobuild.buildroot.org/results/ab2efdd1eac64474adf00d8e60b42110c6e89143



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>