package/libpfm4: disable examples

package/libpfm4: disable examples

Do not build examples to avoid a build failure on rtop example which
depends on ncurses and has been added in version 4.11.0 with
https://sourceforge.net/p/perfmon2/libpfm4/ci/7d1ec562ddbb2af2d6b7af8cec78b03213a3e4ce

/tmp/instance-0/output-1/host/bin/arm-linux-gcc   -I. -I/tmp/instance-0/output-1/build/libpfm4-4.11.0/perf_examples/../include -DCONFIG_PFMLIB_DEBUG -DCONFIG_PFMLIB_OS_LINUX -I. -D_GNU_SOURCE -pthread -o rtop  rtop.o perf_util.o /tmp/instance-0/output-1/build/libpfm4-4.11.0/perf_examples/../lib/libpfm.a  -lpthread -lncurses -ltinfo -lm
/tmp/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-gnueabihf/9.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: cannot find -ltinfo

Fixes:
 - http://autobuild.buildroot.org/results/8ad23a82bcf32cd22256f9984a8f4cfa45951eee

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>