package/mbedtls: disable MBEDTLS_FATAL_WARNINGS

Disable MBEDTLS_FATAL_WARNINGS to avoid the following build failure
since bump to version 2.28.0 in commit
0f8aab08e762742d753d5920718f31c629d0f8d7:

/home/buildroot/autobuild/instance-0/output-1/host/arm-buildroot-linux-gnueabihf/sysroot/usr/include/features.h:412:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Werror=cpp]
  412 | #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
      |    ^~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/af017382bb5772de7383d71ed7b8af44717981f9

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mbedtls/mbedtls.mk

@@ -8,7 +8,8 @@ MBEDTLS_VERSION = 2.28.0
 MBEDTLS_SITE = $(call github,ARMmbed,mbedtls,v$(MBEDTLS_VERSION))
 MBEDTLS_CONF_OPTS = \
 	-DENABLE_PROGRAMS=$(if $(BR2_PACKAGE_MBEDTLS_PROGRAMS),ON,OFF) \
-	-DENABLE_TESTING=OFF
+	-DENABLE_TESTING=OFF \
+	-DMBEDTLS_FATAL_WARNINGS=OFF
 MBEDTLS_INSTALL_STAGING = YES
 MBEDTLS_LICENSE = Apache-2.0
 MBEDTLS_LICENSE_FILES = LICENSE