When openwrt toolchain uses eglibc (2.15) instead of uclibc,

When openwrt toolchain uses eglibc (2.15) instead of uclibc,
libevent2 build fails to following error:

Package libevent2 is missing dependencies for the following libraries:
librt.so.1

Fix is to define
DEPENDS:=+USE_EGLIBC:librt
which adds the librt dependency only if the eglibc is used by toolchain.
Patch has been tested by building the libevent2 both with the the eglibc and
uclibc  openwrt toolchains.

Signed-off-by: Mika Laitio <mika.laitio@nokia.com>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@33007 3c298f89-4303-0410-b956-a3cf2f4a3e73