Set loadAcquire() as const

Fixes powerpc build.

Fetched-from: https://bugzilla.redhat.com/attachment.cgi?id=812643
Signed-off-by: Fatih Aşıcı <fatih.asici@gmail.com>

Index: qtbase-opensource-src-5.1.1/src/corelib/thread/qoldbasicatomic.h
===================================================================
--- qtbase-opensource-src-5.1.1.orig/src/corelib/thread/qoldbasicatomic.h
+++ qtbase-opensource-src-5.1.1/src/corelib/thread/qoldbasicatomic.h
@@ -63,7 +63,7 @@ public:
     // Atomic API, implemented in qatomic_XXX.h
 
     int load() const { return _q_value; }
-    int loadAcquire() { return _q_value; }
+    int loadAcquire() const { return _q_value; }
     void store(int newValue) { _q_value = newValue; }
     void storeRelease(int newValue) { _q_value = newValue; }
 
@@ -107,7 +107,7 @@ public:
     // Atomic API, implemented in qatomic_XXX.h
 
     T *load() const { return _q_value; }
-    T *loadAcquire() { return _q_value; }
+    T *loadAcquire() const { return _q_value; }
     void store(T *newValue) { _q_value = newValue; }
     void storeRelease(T *newValue) { _q_value = newValue; }