Change include path to fix error with embedded cmake builds

[heshpdx]

Fixed a build issue. I integrate the capstone directory into a larger project and use the cmakefile. It was an easy fix, but not sure how I can show how to reproduce in your git repo.

In file included from /home/mjm/sim/src/capstone/include/capstone/capstone.h:302,
                 from /home/mjm/sim/src/ampere/event_stream/event_stream.cc:22:
/home/mjm/sim/src/capstone/include/capstone/riscv.h:16:10: fatal error: capstone/platform.h: No such file or directory
#include "capstone/platform.h"
          ^~~~~~~~~~~~~~~~~~~~~
compilation terminated.

[aquynh]

merged, thanks!