Disable ignored-qualifier warning in SDK (earlephilhower#238)

The Pico SDK has some magic with const pointers that generates lots of
"ignored qualifer" warnings on the more pedantic modes.  To clean the
normal builds up, disable this warning for now.  At some point a PR to
the PICO-SDK may be indicated.

Author: earlephilhower

platform.txt

@@ -35,8 +35,8 @@ compiler.path={runtime.tools.pqt-gcc.path}/bin/
 compiler.warning_flags=-Werror=return-type
 compiler.warning_flags.none=-Werror=return-type
 compiler.warning_flags.default=-Werror=return-type
-compiler.warning_flags.more=-Wall -Werror=return-type
-compiler.warning_flags.all=-Wall -Wextra -Werror=return-type
+compiler.warning_flags.more=-Wall -Werror=return-type -Wno-ignored-qualifiers
+compiler.warning_flags.all=-Wall -Wextra -Werror=return-type -Wno-ignored-qualifiers
 
 compiler.defines={build.led} {build.usbstack_flags} -DCFG_TUSB_MCU=OPT_MCU_RP2040 -DUSB_VID={build.vid} -DUSB_PID={build.pid} '-DUSB_MANUFACTURER={build.usb_manufacturer}' '-DUSB_PRODUCT={build.usb_product}'
 compiler.includes="-iprefix{runtime.platform.path}/" "@{runtime.platform.path}/lib/platform_inc.txt"