libpfm4: fix static-only build

By default, libpfm4 tries to build a shared library. However, by
passing CONFIG_PFMLIB_SHARED=n, we can convince it to build only a
static library. Therefore, this commit uses that to fix the
static-only build, therefore fixing:

  http://autobuild.buildroot.net/results/efa/efae782912face7c36417dfb1023228d6b137654/



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