fix(plat/st): add STM32IMAGE_SRC

The dependency on this macro was added by patch [1]. But the macro
itself was forgotten in the patch.

 [1] 128e0b3e

 ("stm32mp1: update rules for stm32image tool")

Change-Id: I49219e1e13828b97b95f404983da33ef4567fe23
Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>

plat/st/stm32mp1/platform.mk

@@ -64,6 +64,7 @@ endif
 # Variables for use with stm32image
 STM32IMAGEPATH		?= tools/stm32image
 STM32IMAGE		?= ${STM32IMAGEPATH}/stm32image${BIN_EXT}
+STM32IMAGE_SRC		:= ${STM32IMAGEPATH}/stm32image.c
 
 # Enable flags for C files
 $(eval $(call assert_booleans,\