libfdt: avoid shadowing "err" in FDT_CHECK_HEADER

FDT_CHECK_HEADER declares an internal variable named "err" whose name is
far too generic and will produce the following -Wshadow warnings:

libfdt/fdt_ro.c: In function 'fdt_node_offset_by_compatible':
libfdt/fdt_ro.c:555:2: error: declaration of 'err' shadows a previous
local [-Werror=shadow]
libfdt/fdt_ro.c:553:14: error: shadowed declaration is here
[-Werror=shadow]
cc1: all warnings being treated as errors

Since this variable is only used internally in the macro, rename to
__err which should be prefixed enough not to cause new shadow warnings.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

libfdt/libfdt_internal.h

@@ -57,9 +57,9 @@
 
 #define FDT_CHECK_HEADER(fdt) \
 	{ \
-		int err; \
-		if ((err = fdt_check_header(fdt)) != 0) \
-			return err; \
+		int __err; \
+		if ((__err = fdt_check_header(fdt)) != 0) \
+			return __err; \
 	}
 
 int _fdt_check_node_offset(const void *fdt, int offset);