Merge #17563: lib: fix a compiler warning: unused GetDevURandom()

ca2e474372 Fix a compiler warning: unused GetDevURandom() (Vasil Dimov)

Pull request description:

  ~~Only define GetDevURandom() if it is going to be used.~~

  Silence by planting a dummy reference to the `GetDevURandom` symbol
  in the places where we don't call the function.

ACKs for top commit:
  practicalswift:
    ACK ca2e474372 -- increased signal to noise in compiler diagnostics is good
  sipa:
    utACK ca2e474372
  hebasto:
    re-ACK ca2e474372, tested on macOS 10.15.6 + llvm clang 10.0.0

Tree-SHA512: 03c98f00dad5d9a3c5c9f68553d72ad5489ec02f18b9769108a22003ec7be7819a731b1eab6a9f64dafb5be0efddccf6980de7e3bb90cd20d4f4d72f74124675

src/random.cpp

@@ -315,12 +315,16 @@ void GetOSRand(unsigned char *ent32)
     if (getentropy(ent32, NUM_OS_RANDOM_BYTES) != 0) {
         RandFailure();
     }
+    // Silence a compiler warning about unused function.
+    (void)GetDevURandom;
 #elif defined(HAVE_GETENTROPY_RAND) && defined(MAC_OSX)
     /* getentropy() is available on macOS 10.12 and later.
      */
     if (getentropy(ent32, NUM_OS_RANDOM_BYTES) != 0) {
         RandFailure();
     }
+    // Silence a compiler warning about unused function.
+    (void)GetDevURandom;
 #elif defined(HAVE_SYSCTL_ARND)
     /* FreeBSD, NetBSD and similar. It is possible for the call to return less
      * bytes than requested, so need to read in a loop.
@@ -334,6 +338,8 @@ void GetOSRand(unsigned char *ent32)
         }
         have += len;
     } while (have < NUM_OS_RANDOM_BYTES);
+    // Silence a compiler warning about unused function.
+    (void)GetDevURandom;
 #else
     /* Fall back to /dev/urandom if there is no specific method implemented to
      * get system entropy for this OS.