storage: Fix some Rust types on x86_64

x86 has c_char defined to signed i8.
Fix the Rust storage build by explicitly casting
some pointers.

Bug: 304851081
Test: Build Trusty generic-x86_64-test project
Change-Id: I4858dc5f5b756c19bdd03a7ad63a9197630b95d6
1 file changed