fix(lwip/cmake): avoid using uninitialized srcs variable

Currently, lwip's CMakeLists.txt might use an uninitialized srcs value
if CONFIG_LWIP_ENABLE is disabled. This means it could use a srcs value
set by another component, involving sources that are entirely
unrelated to the lwip component during registration. Perhaps we
should simply exit when CONFIG_LWIP_ENABLE is not enabled and avoid
registering the component altogether.

Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>

components/lwip/CMakeLists.txt

@@ -1,4 +1,5 @@
 idf_build_get_property(target IDF_TARGET)
+set(srcs "")
 
 if(CONFIG_LWIP_ENABLE)
     if(NOT ${target} STREQUAL "linux")