[compiler-rt] Fix a warning

This patch fixes:

  compiler-rt/lib/builtins/truncxfbf2.c:16:7: error: ISO C requires a
  translation unit to contain at least one declaration
  [-Werror,-Wempty-translation-unit]

Author: kazutakahirata

compiler-rt/lib/builtins/truncxfbf2.c

@@ -14,3 +14,6 @@
 COMPILER_RT_ABI dst_t __truncxfbf2(long double a) { return __truncXfYf2__(a); }
 
 #endif
+
+// Have at least one declaration to suppress warnings.
+enum Unused { ReallyUnused };