charger: add locale global variable

Some functions in minui reference a global extern char *locale.  None
of them are used by charger, but the bfd linker cannot optimize them
out and fails to link charger.  Define char *locale in charger.c
so it can resolve the symbol.

Change-Id: I738daf95d24356bf96bd226d91a5dc64b5d4dc9b

charger/charger.c

@@ -47,6 +47,8 @@
 
 #include "minui/minui.h"
 
+char *locale;
+
 #ifndef max
 #define max(a,b) ((a) > (b) ? (a) : (b))
 #endif