Settings: use size_t for meter count in header

Header.c:150:26: error: implicit conversion loses integer precision: 'int' to 'uint8_t' (aka 'unsigned char') [-Werror,-Wimplicit-int-conversion]
          colSettings->len = len;
                           ~ ^~~

Settings.h

@@ -22,7 +22,7 @@ in the source distribution for its full text.
 #define CONFIG_READER_MIN_VERSION 2
 
 typedef struct {
-   uint8_t len;
+   size_t len;
    char** names;
    int* modes;
 } MeterColumnSetting;