Compiling Solid on a musl-based Linux environment (long story...) gives an error compiling UDisks2 support because it can't find the dev_t type.
Looking up dev_t, it appears that POSIX requires the <sys/types.h> header instead of wherever we're currently finding this from (implicitly?). We already have a sys/types.h include, but we only currently use it for FreeBSD.
Since we're already guarding UDisks2 to Linux-based systems or FreeBSD, I opted just to remove the include guard. This fixes the compile on musl libc and doesn't break compilation on glibc. I'm not aware of any other major C libraries for Linux environments to test against.