Move bn_mul.h to library/

Move `include/mbedtls/bn_mul.h` to `library/bn_mul.h`.

Update includes and references to `bn_mul.h` to new location.

Also remove internal headers from `cpp_dummy_build.cpp` as it should only
test public headers in the library.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 46941e2..c7871eb 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -48,7 +48,7 @@
  * Used in:
  *      library/aria.c
  *      library/timing.c
- *      include/mbedtls/bn_mul.h
+ *      library/bn_mul.h
  *
  * Required by:
  *      MBEDTLS_AESNI_C