Remove incorrect forward-declaration of lgamma_r header

libc++ declares `lgamma_r` as:

```
extern "C" double lgamma_r(double, int *);
```

while modern glibc uses the following:

```
__MATHCALL (lgamma,_r, (_Mdouble_, int *__signgamp));
```

This causes (I did not digged to the very bottom of it) the following compilation error when compiling with nvcc:

```
libcxx/include/__random/binomial_distribution.h(117): error: linkage specification is incompatible with previous "lgamma_r"
/usr/include/x86_64-linux-gnu/bits/mathcalls.h(271): here
```

Author: georgthegreat

libcxx/include/__random/binomial_distribution.h

@@ -97,10 +97,6 @@ class _LIBCPP_TEMPLATE_VIS binomial_distribution {
   }
 };
 
-#ifndef _LIBCPP_MSVCRT_LIKE
-extern "C" double lgamma_r(double, int*);
-#endif
-
 inline _LIBCPP_HIDE_FROM_ABI double __libcpp_lgamma(double __d) {
 #if defined(_LIBCPP_MSVCRT_LIKE)
   return lgamma(__d);