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

Fixes:

  microtek.c: In function ‘MDBG_INIT’:
  microtek.c:163:3: error: unknown type name ‘va_list’
    163 |   va_list ap;
        |   ^~~~~~~
  microtek.c:78:1: note: ‘va_list’ is defined in header ‘<stdarg.h>’; did you forget to ‘#include <stdarg.h>’?
     77 | #include "microtek.h"
    +++ |+#include <stdarg.h>
     78 |
  microtek.c:164:3: warning: implicit declaration of function ‘va_start’; did you mean ‘sane_start’? [-Wimplicit-function-declaration]
    164 |   va_start(ap, format);
        |   ^~~~~~~~
        |   sane_start
  microtek.c:165:54: warning: passing argument 4 of ‘vsnprintf’ makes pointer from integer without a cast [-Wint-conversion]
    165 |   vsnprintf(_mdebug_string, MAX_MDBG_LENGTH, format, ap);
        |                                                      ^~
        |                                                      |
        |                                                      int
  In file included from ../include/sane/sanei_config.h:50,
                   from microtek.c:70:
  .../host/x86_64-buildroot-linux-uclibc/sysroot/usr/include/stdio.h:359:57: note: expected ‘__va_list_tag *’ but argument is of type ‘int’
    359 |         const char *__restrict __format, __gnuc_va_list __arg)
        |                                          ~~~~~~~~~~~~~~~^~~~~
  microtek.c:166:3: warning: implicit declaration of function ‘va_end’ [-Wimplicit-function-declaration]
    166 |   va_end(ap);
        |   ^~~~~~

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

backend/microtek.c

@@ -57,6 +57,7 @@
 
 #include "../include/sane/config.h"
 
+#include <stdarg.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>