Remove unused `#include <stdarg.h>` in include/rocksdb/c.h (#11302)

Summary:
This include is unused in the header. In one build environment of ours, stdarg.h is actually not present, and this include prevents us from building rocksdb dependencies.

We're currently monkey-patching this line out in our build script (still WIP), which of course is not good. ec2852caa3

Note that removing this include might break builds in unexpected ways that include rocksdb/c.h and then use `va_start`, `va_end`, etc. However, if you're using these functions, you really should include stdarg.h yourself, so I don't think this should prevent this PR.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11302

Reviewed By: ajkr

Differential Revision: D44139819

Pulled By: cbi42

fbshipit-source-id: 10c40b0b0260b23ccb7dc84e55a993c7dfbdc4cf

include/rocksdb/c.h

@@ -62,7 +62,6 @@
 extern "C" {
 #endif
 
-#include <stdarg.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>