[libc][Obvious] Include <fenv.h> from DummyFenv.h.

GitOrigin-RevId: 9ad2091e78eb47e6707abbc7c83e208ea1150589
diff --git a/utils/FPUtil/DummyFEnv.h b/utils/FPUtil/DummyFEnv.h
index e303f6c..4f4c2c0 100644
--- a/utils/FPUtil/DummyFEnv.h
+++ b/utils/FPUtil/DummyFEnv.h
@@ -9,6 +9,7 @@
 #ifndef LLVM_LIBC_UTILS_FPUTIL_DUMMY_FENV_H
 #define LLVM_LIBC_UTILS_FPUTIL_DUMMY_FENV_H
 
+#include <fenv.h>
 #include <math.h>
 
 namespace __llvm_libc {