fix(esp_phy/cmake): initialize embed_files variable

In cmakev2, a component can be evaluated within the context of another
component, so it's important to properly initialize each variable used
by the component.

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

components/esp_phy/CMakeLists.txt

@@ -10,6 +10,8 @@ if(IDF_TARGET STREQUAL "esp32p4")
     return()
 endif()
 
+set(embed_files)
+
 if(CONFIG_ESP_PHY_ENABLED)
 
     set(srcs "src/phy_override.c" "src/lib_printf.c" "src/phy_common.c")