Only rename fgetpos/fsetpos/fseeko/ftello/funopen if we're N or newer.

Without this, setting __FILE_OFFSET_BITS to 64 and targeting pre-L
made these functions entirely unavailable.

Bug: android/ndk#333
Test: builds
Change-Id: Id17ae3c070f8b2650a9bc9aa2aa2e92c5fcdf4ad

libc/include/stdio.h

@@ -176,7 +176,7 @@ int renameat(int, const char*, int, const char*);
 int fseek(FILE*, long, int);
 long ftell(FILE*);
 
-#if defined(__USE_FILE_OFFSET64)
+#if defined(__USE_FILE_OFFSET64) && __ANDROID_API__ >= __ANDROID_API_N__
 int fgetpos(FILE*, fpos_t*) __RENAME(fgetpos64);
 int fsetpos(FILE*, const fpos_t*) __RENAME(fsetpos64);
 int fseeko(FILE*, off_t, int) __RENAME(fseeko64);