newlib.mk: fix NEWLIB_INCLUDE_DIR not being set by PATTERNS

As NEWLIB_INCLUDE_DIR has already been set here, with an empty value, it is not
overwriting it because of the '?='.

makefiles/libc/newlib.mk

@@ -52,7 +52,7 @@ ifeq (,$(NEWLIB_INCLUDE_DIR))
 # the patterns above. We use the -isystem gcc/clang argument to add the include
 # directories as system include directories, which means they will not be
 # searched until after all the project specific include directories (-I/path)
-  NEWLIB_INCLUDE_DIR ?= $(firstword $(dir $(wildcard $(addsuffix /newlib.h, $(NEWLIB_INCLUDE_PATTERNS)))))
+  NEWLIB_INCLUDE_DIR := $(firstword $(dir $(wildcard $(addsuffix /newlib.h, $(NEWLIB_INCLUDE_PATTERNS)))))
 endif
 
 # If nothing was found we will try to fall back to searching for a cross-gcc in