eliminate variable-length array in dr_frontend_unix.c

Some build environments may want to link drfrontendlib into a C++ app by directly compiling the frontendlib sources, and may use -Wvla to build dr_frontend_unix.c, which then hits:

libutil/dr_frontend_unix.c:135:13: error: variable length array used [-Werror,-Wvla]
char tmp[full_path_size];

That's legal in C99 but not in C++.

Not high priority since this does not affect officially supported build actions but we may want to clean it up for maximum compatibility.