package/apr: Add libuuid as optional dependency

libuuid is picked up by apr when present:

Checking for OS UUID Support...
checking uuid/uuid.h usability... yes
checking uuid/uuid.h presence... yes
checking for uuid/uuid.h... yes
checking for library containing uuid_generate... -luuid
checking for uuid_generate... yes
checking for os uuid usability... yes

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a output/target/usr/lib/libapr-1.so | grep libuuid
 0x00000001 (NEEDED)                     Shared library: [libuuid.so.1]

[Thomas: move the added chunk a bit further down, and use += instead
of =.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>