ZGW-3429: build: Add path for generated pkgconfig for libs2

When libs2 is a submodule it can't be updated with previous patch (see links).
For zgw public repo libs2 is code dropped, until it is maintained in public.

This change is not really helpful here,
impact is low but it help to align other tree.

Relate-to: ee22272#r145562119
Relate-to: #24
Bug-SiliconLabs: ZGW-3429
Origin: #29
Signed-off-by: Philippe Coval <[email protected]>

Author: rzr

CMakeLists.txt

@@ -92,6 +92,7 @@ set(CONTIKI_DIR ${CMAKE_CURRENT_SOURCE_DIR}/contiki)
 set(ZGW_SRC_DIR ${CMAKE_SOURCE_DIR}/src)
 
 # Include lib S2
+include_directories( ${CMAKE_CURRENT_BINARY_DIR} )
 set(LIBS2 "${CMAKE_CURRENT_SOURCE_DIR}/libs2" )
 
 set(S2_LIBRARIES s2_controller s2crypto aes)