gloo/common/aligned_allocator.h: switch to the one-arg allocate function, dropping const_pointer reference

Summary:
This avoids the following errors:

  buck-out/v2/gen/fbcode/d5479947763d4841/gloo/common/__common__/headers/gloo/common/aligned_allocator.h:53:38: error: no type named 'const_pointer' in 'std::allocator<void>'

Reviewed By: r-barnes

Differential Revision: D37416148

fbshipit-source-id: 9dd57b43716046b4d1c6535f2b68360154eb8fda

Author: meyering

gloo/common/aligned_allocator.h

@@ -48,9 +48,7 @@ class aligned_allocator {
     return &r;
   }
 
-  inline pointer allocate(
-      size_type sz,
-      typename std::allocator<void>::const_pointer = 0) {
+  inline pointer allocate(size_type sz) {
     pointer p;
     if (posix_memalign(
             reinterpret_cast<void**>(&p), ALIGNMENT, sizeof(T) * sz)) {