libmbedtls: fix unrecognized compiler option

Prior to this patch when compiling with clang there an error:
error: unknown warning option '-Wno-unused-functions'; did you mean '-Wno-unused-function'? [-Werror,-Wunknown-warning-option]

Fix this by using -Wno-unused-function instead.

Fixes: dfafe50 ("Import mbedtls-3.4.0")
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

lib/libmbedtls/sub.mk

@@ -134,7 +134,7 @@ srcs-$(sm-$(ta-target)) += $(addprefix mbedtls/library/, $(SRCS_TLS))
 cflags-lib-y += -Wno-redundant-decls
 cflags-lib-y += -Wno-switch-default
 cflags-lib-y += -Wno-declaration-after-statement
-cflags-lib-y += -Wno-unused-functions
+cflags-lib-y += -Wno-unused-function
 
 ifeq ($(CFG_CRYPTOLIB_NAME_mbedtls),y)
 subdirs-$(sm-core) += core