ltp: Fix cross-compilation

Compilation included the flag "-I/usr/include/tirpc" which ignores
sysroot. This makes a cross-compile to suddenly use host files rather
than the files installed in the compiler sysroot.

Signed-off-by: Mats Liljegren <mats.liljegren@enea.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
Tested-by: Mats Liljegren <mats.liljegren@enea.com>