Fix debug build broken issue.

Fix the following error:
/usr/bin/ld: ../../lib/libopencv_ts.a(ts_gtest.cpp.o): relocation
R_X86_64_32 against `.rodata' can not be used when making a shared object;
recompile with -fPIC
../../lib/libopencv_ts.a: could not read symbols: Bad value
collect2: error: ld returned 1 exit status
make[2]: *** [lib/cv2.so] Error 1
make[1]: *** [modules/python2/CMakeFiles/opencv_python2.dir/all] Error 2
make: *** [all] Error 2