Fix ambiguous definition of atime/mtime/ctime on alpha

One of the implementations is enabled when the stat struct has
::st_atim member, another — when it has ::st_atimensec member.

On alpha, the stat struct has both members, defined as union here:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/alpha/bits/stat.h#l48

and then used here:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/alpha/bits/stat.h#l92

This commit forcefully disables the second implementation on alpha.

Change-Id: Ifc284d72b68b9bac590b518f31960288df3a087d
Done-with: Michael Cree <mcree@orcon.net.nz>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qfilesystemengine_unix.cpp

@@ -285,7 +285,7 @@ mtime(const T &statBuffer, int)
 { return timespecToMSecs(statBuffer.st_mtimespec); }
 #endif
 
-#ifndef st_mtimensec
+#if !defined(st_mtimensec) && !defined(__alpha__)
 // Xtimensec
 template <typename T>
 Q_DECL_UNUSED static typename std::enable_if<(&T::st_atimensec, true), qint64>::type