libevdev: fix shared only build

Do not build test-static-link in case of shared only build,
fixes the following compile failure:

libtool: link: [..]/host/usr/bin/arm-buildroot-linux-uclibcgnueabi-gcc -std=gnu99 -I.. -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -static -o test-static-link test_static_link-test-link.o  ../libevdev/.libs/libevdev.so -Wl,-rpath -Wl,[...]/build/libevdev-1.4/libevdev/.libs
[...]/arm-buildroot-linux-uclibcgnueabi/bin/ld: attempted static link of dynamic object `../libevdev/.libs/libevdev.so'
collect2: error: ld returned 1 exit status
Makefile:719: recipe for target 'test-static-link' failed

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>