revert bullet include path variable to fix ubuntu build

fggl/phys/bullet/CMakeLists.txt

@@ -11,7 +11,7 @@ else()
     else()
         # FIXME: this shouldn't be necessary, for modern cmake, linking the libraries should be enough
         target_compile_definitions(fggl PUBLIC ${BULLET_DEFINITIONS})
-        target_include_directories(fggl PUBLIC ${BULLET_INCLUDE_DIRS})
+        target_include_directories(fggl PUBLIC ${BULLET_INCLUDE_DIR})
         target_link_libraries(fggl PUBLIC ${BULLET_LIBRARIES})
     endif()