Fix compiler warning for -Wmissing-variable-declarations part2

静的でないグローバル変数に変数宣言がないとclangに指摘されたため配列変数をconstexpr変数に変更してコンパイラー警告を修正します。

clang-17のレポート (file pathを一部省略)

src/icons/iconfiles.h:15:10: warning: no previous extern declaration for non-static variable 'iconfiles' [-Wmissing-variable-declarations]
   15 |     char iconfiles[][ MAX_ICON_FILES ]={
      |          ^
src/control/controllabel.h:24:10: warning: no previous extern declaration for non-static variable 'mode_label' [-Wmissing-variable-declarations]
   24 |     char mode_label[][ MAX_CONTROL_LABEL ] ={
      |          ^
src/control/keysyms.h:23:13: warning: no previous extern declaration for non-static variable 'keysyms' [-Wmissing-variable-declarations]
   23 |     KEYSYMS keysyms[] ={
      |             ^
src/control/controllabel.h:24:5: note: declare 'static' if the variable is not intended to be used outside of this translation unit
   24 |     char mode_label[][ MAX_CONTROL_LABEL ] ={
      |     ^
src/browsers.cpp:14:10: warning: no previous extern declaration for non-static variable 'browsers' [-Wmissing-variable-declarations]
   14 |     char browsers[ BROWSER_NUM ][ 2 ][ MAX_TEXT ]={
      |          ^

JDimproved / JDim

Fix compiler warning for -Wmissing-variable-declarations part2 #1385