action_render_string_filters_template(): Declare a variable at the beginning

... of the function to silence a compiler warning when building with -std=c89:

   cc -c -pipe -fstack-protector-all -ggdb -Wshadow  -Wconversion -I/usr/local/include/  -pthread -Wall -std=c89   cgiedit.c -o cgiedit.o
   cgiedit.c:4436:9: warning: GCC does not allow variable declarations in for loop initializers before C99 [-Wgcc-compat]
      for (int i=0; i < SZ(desc); ++i)
           ^
   1 warning generated.