Fix: Stream: Replace fflush() with fseek()

This is a follow-up to PR #8331; fixing the Stream methods not covered
there.

Author: Filip Jagodzinski

platform/Stream.cpp

@@ -147,7 +147,7 @@ int Stream::printf(const char *format, ...)
     lock();
     std::va_list arg;
     va_start(arg, format);
-    fflush(_file);
+    std::fseek(_file, 0, SEEK_CUR);
     int r = vfprintf(_file, format, arg);
     va_end(arg);
     unlock();
@@ -169,7 +169,7 @@ int Stream::scanf(const char *format, ...)
 int Stream::vprintf(const char *format, std::va_list args)
 {
     lock();
-    fflush(_file);
+    std::fseek(_file, 0, SEEK_CUR);
     int r = vfprintf(_file, format, args);
     unlock();
     return r;