link.c: fixed to include <sys/types.h> for caddr_t

When building against musl libc compiling bails out because of caddr_t not
being declared:
--------------------------------------------------------------------------
link.c: In function 'wireless_sigqual':
link.c:41:24: error: 'caddr_t' undeclared (first use in this function)
--------------------------------------------------------------------------

As caddr_t is declared in <sys/types.h> this header needs to be included.
build: add DESTDIR support

This change implements support for specifying an alternative installation
destination directory by setting DESTDIR when calling 'make install'.

Example: make DESTDIR=/alternative-rootfs install

It is particularly useful when cross-compiling and installing in a dedicated
directory where for example the rootfs or a build sandbox for the target
architecture is mounted.

By using DESTDIR you can avoid to mess with PREFIX to include the alternative
installation directory!