Add 'nonstring' attribute to firmware_info magic field.

Fixes compiler warnings such as:

firmware_info.c:49:18: warning: initializer-string for array of 'char'
truncates NUL terminator but destination lacks 'nonstring' attribute (9
chars into 8 available) [-Wunterminated-string-initialization]
   49 |         .magic = "HACKRFFW",
      |                  ^~~~~~~~~~

firmware/common/firmware_info.h

@@ -25,7 +25,7 @@
 #include <stdint.h>
 
 struct firmware_info_t {
-	char magic[8];
+	__attribute__((nonstring)) char magic[8];
 	uint16_t struct_version;
 	uint16_t dfu_mode;
 	uint32_t supported_platform;