newlib.mk: fix: do not require slash terminated NEWLIB_INCLUDE_DIR

When NEWLIB_INCLUDE_DIR is set from other parts than 'COMPILER_INCLUDE_PATHS' it
does not have a trailing slash.
Also, it makes it more problematic when supplying it from the command line.

And anyway having two '/' does not break anything.

makefiles/libc/newlib.mk

@@ -79,9 +79,9 @@ ifeq ($(TOOLCHAIN),llvm)
 endif
 
 ifeq (1,$(USE_NEWLIB_NANO))
-  NEWLIB_NANO_INCLUDE_DIR ?= $(firstword $(wildcard $(NEWLIB_INCLUDE_DIR)newlib-nano \
-                                                    $(NEWLIB_INCLUDE_DIR)newlib/nano \
-                                                    $(NEWLIB_INCLUDE_DIR)nano))
+  NEWLIB_NANO_INCLUDE_DIR ?= $(firstword $(wildcard $(NEWLIB_INCLUDE_DIR)/newlib-nano \
+                                                    $(NEWLIB_INCLUDE_DIR)/newlib/nano \
+                                                    $(NEWLIB_INCLUDE_DIR)/nano))
   # newlib-nano overrides newlib.h and its include dir should therefore go before
   # the regular system include dirs.
   INCLUDES := -isystem $(NEWLIB_NANO_INCLUDE_DIR) $(INCLUDES)