backend/sm3600-scanutil: fix uclibc compile (include stdarg.h for va_list/va_start/va_end)

Fixes:

  sm3600-scanutil.c: In function ‘debug_printf’:
  sm3600-scanutil.c:69:3: error: unknown type name ‘va_list’
     69 |   va_list ap;
        |   ^~~~~~~
  sm3600-scanutil.c:48:1: note: ‘va_list’ is defined in header ‘<stdarg.h>’; did you forget to ‘#include <stdarg.h>’?
     47 | #include "sm3600-scantool.h"
    +++ |+#include <stdarg.h>
     48 |
  sm3600-scanutil.c:75:3: warning: implicit declaration of function ‘va_start’; did you mean ‘sane_start’? [-Wimplicit-function-declaration]
     75 |   va_start(ap,szFormat);
        |   ^~~~~~~~
        |   sane_start
  sm3600-scanutil.c:76:28: warning: passing argument 3 of ‘vfprintf’ makes pointer from integer without a cast [-Wint-conversion]
     76 |   vfprintf(stderr,szFormat,ap);
        |                            ^~
        |                            |
        |                            int
  In file included from ../include/sane/sanei_config.h:50,
                   from sm3600.c:70:
  .../host/x86_64-buildroot-linux-uclibc/sysroot/usr/include/stdio.h:339:23: note: expected ‘__va_list_tag *’ but argument is of type ‘int’
    339 |        __gnuc_va_list __arg);
        |        ~~~~~~~~~~~~~~~^~~~~
  In file included from sm3600.c:94:
  sm3600-scanutil.c:77:3: warning: implicit declaration of function ‘va_end’ [-Wimplicit-function-declaration]
     77 |   va_end(ap);
        |   ^~~~~~

Signed-off-by: Peter Seiderer <ps.report@gmx.net>

backend/sm3600-scanutil.c

@@ -43,6 +43,7 @@ Userspace scan tool for the Microtek 3600 scanner
 
 ====================================================================== */
 
+#include <stdarg.h>
 #include <unistd.h>
 #include "sm3600-scantool.h"