tools: prepend DESTDIR when installing

Some tools prepend DESTDIR properly, some not. ktest-tool and
klee-stats do not, so 'make install' chokes with an error:
llvm[2]: Installing Release+Asserts /usr/bin/ktest-tool
/usr/bin/install: cannot create regular file '/usr/bin/ktest-tool': Permission denied

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Merge pull request #177 from jirislaby/destdir

tools: prepend DESTDIR when installing