opkg: fix header installation for libopkg

opkg: fix header installation for libopkg

When opkg is built outside the source directory, libopkg headers aren't all
installed as the search path for headers only includes the directory that make
is being ran in, not the source directory. So we fix this by adding
$(srcdir)/*.h to the list of header files to install.

Without this patch, "/usr/include/libopkg" will only include the file config.h.
With this patch, other files such as "opkg.h" are present which are needed to
compile software which uses libopkg.

The new patch has also been submitted upstream as a fix to opkg itself.

(From OE-Core rev: 873689bbabba25e7be5c12317c04519a7bc8d0ef)

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>