omnia-mcutool: build as C99

GCC15 defaults to C23, which does not work here.

Signed-off-by: Rosen Penev <[email protected]>
Link: openwrt/openwrt#23042
Signed-off-by: Robert Marko <[email protected]>

Author: neheb

package/utils/omnia-mcutool/Makefile

@@ -36,6 +36,7 @@ microcontroller on the Turris Omnia router. It can also show state of MCU
 settings and configure MCU options (GPIOs, LEDs, power).
 endef
 
+TARGET_CFLAGS += -std=gnu99
 TARGET_LDFLAGS += -lcrypto
 
 define Build/Compile