Make OpenSSLGenerator max() and min() constexpr

The latest version of gcc seems to be enforcing that these are static
and constexpr, presumably because std::random_device is now using them
in a constexpr context.  It seems odd this would be required (you'd
think it would be breaking backward compatibility) but it's an easy
enough change to make.

Tested:
Only tested that code builds.  Not tested functionally.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I3f5e7077d56178fbc7b4e02f4e2f4a357c928c5b
diff --git a/include/random.hpp b/include/random.hpp
index d11c249..0e63391 100644
--- a/include/random.hpp
+++ b/include/random.hpp
@@ -20,11 +20,11 @@
         return index;
     }
 
-    uint8_t max()
+    static constexpr uint8_t max()
     {
         return std::numeric_limits<uint8_t>::max();
     }
-    uint8_t min()
+    static constexpr uint8_t min()
     {
         return std::numeric_limits<uint8_t>::min();
     }