package/uftrace: needs dynamic library

Since its addition in commit 09c97972,
uftrace always builds a dynamic library which will raise the following
build failure:

  LINK     libmcount/libmcount.so
.../ld: .../crtbeginT.o: relocation R_X86_64_32 against hidden symbol `__TMC_END__' can not be used when making a shared object

Fixes:
 - http://autobuild.buildroot.org/results/980bb38c6417bd48828379677762382030c5b28a



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>