x265: update INSTALL_PREFIX

cmake defaults the install directory to /usr/local on UNIX.
- https://cmake.org/cmake/help/latest/variable/CMAKE_INSTALL_PREFIX.html

This results is the x265.pc file located in /usr/local/lib/pkgconfig
instead of /usr/lib/pkgconfig and not being found when pkg-config is
called.

packages/addons/addon-depends/ffmpegx-depends/x265/package.mk

@@ -14,5 +14,5 @@ PKG_TOOLCHAIN="make"
 
 pre_configure_target() {
   LDFLAGS+=" -ldl"
-  cmake -G "Unix Makefiles" ./source
+  cmake -DCMAKE_INSTALL_PREFIX=/usr -G "Unix Makefiles" ./source
 }