LibC: Implement __fpending

Some gnu programs want this and I don't want to make FILE non-opaque

userspace/libraries/LibC/include/stdio.h

@@ -127,6 +127,7 @@ int		vsnprintf(char* __restrict s, size_t n, const char* __restrict format, va_l
 int		vsprintf(char* __restrict s, const char* __restrict format, va_list ap);
 int		vsscanf(const char* __restrict s, const char* __restrict format, va_list arg);
 
+size_t __fpending(FILE* stream);
 void __fseterr(FILE* stream);
 
 __END_DECLS

userspace/libraries/LibC/stdio.cpp

@@ -134,6 +134,12 @@ void clearerr(FILE* file)
 	file->error = false;
 }
 
+size_t __fpending(FILE* file)
+{
+	ScopeLock _(file);
+	return file->buffer_rd_size ? 0 : file->buffer_idx;
+}
+
 void __fseterr(FILE* file)
 {
 	ScopeLock _(file);