Fix warnings seen on SDCC 4.6.0 due to missing const qualifiers

Building with SDCC 4.6.0 emits tons of warnings e.g.:

	cmd_parser.c:31: warning 356: object in read-only code space should be const
	cmd_parser.c:36: warning 357: pointer to object in read-only code space should be pointer to const

These of course become fatal when building with --Werror.

Comparing resultant binaries, it seems that a single string is shifted
(deduplication?) but the numbers emitted at the end of the build are
otherwise identical.

httpd/page_impl.h

@@ -3,7 +3,7 @@
 
 #include <stdbool.h>
 
-extern __code uint8_t * __code HTTP_RESPONCE_TXT;
+extern __code const uint8_t * __code const HTTP_RESPONCE_TXT;
 
 bool send_counters(uint8_t phys_port);
 void send_status(void);