ltp-testsuite: fix build breakage with libcap

ltp-testsuite needs libcap with file attributes which in turn needs
attr.
Only libcap is checked for so if it lacks file attr support it breaks.
Disable libcap support unless it's got attr support, fixes:
http://autobuild.buildroot.net/results/b16b20963c7cb17d8e220a5baa1356372ebf85a6/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>