include/dfu: rearrange for standard use of extern "C" in private headers

Consistently place C++ use of extern "C" after all include directives,
within the negative branch of _ASMLANGUAGE if used.

Background from issue #17997:

Declarations that use C linkage should be placed within extern "C"
so the language linkage is correct when the header is included by
a C++ compiler.

Similarly #include directives should be outside the extern "C" to
ensure the language-specific default linkage is applied to any
declarations provided by the included header.

See: https://en.cppreference.com/w/cpp/language/language_linkage
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

include/dfu/flash_img.h

@@ -8,12 +8,12 @@
 #ifndef ZEPHYR_INCLUDE_DFU_FLASH_IMG_H_
 #define ZEPHYR_INCLUDE_DFU_FLASH_IMG_H_
 
+#include <storage/flash_map.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include <storage/flash_map.h>
-
 struct flash_img_context {
 	u8_t buf[CONFIG_IMG_BLOCK_BUF_SIZE];
 	const struct flash_area *flash_area;