xen: common: Use unbounded array for symbols_offset.

Using a singleton array causes gcc5 to report:
symbols.c: In function 'symbols_lookup':
symbols.c:128:359: error: array subscript is above array bounds [-Werror=array-bounds]
symbols.c:136:176: error: array subscript is above array bounds [-Werror=array-bounds]

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/common/symbols.c

@@ -19,7 +19,7 @@
 #include <xen/spinlock.h>
 
 #ifdef SYMBOLS_ORIGIN
-extern const unsigned int symbols_offsets[1];
+extern const unsigned int symbols_offsets[];
 #define symbols_address(n) (SYMBOLS_ORIGIN + symbols_offsets[n])
 #else
 extern const unsigned long symbols_addresses[];