oprofile: don't install shared library in BR2_STATIC_LIBS=y configs

When building with BR2_STATIC_LIBS=y, no shared library gets built, so
the installation of libopagent.so* fails. This commit gets rid of this
installation step when BR2_STATIC_LIBS=y.

Fixes:

  http://autobuild.buildroot.net/results/2a6604622392a3f127fb61505e2751a88a90af7b/



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