gcc: Use <diagnostic.h>, not <diagnostic-core.h>.

* gcc-plugin/src/c-expr.y, gcc-plugin/src/tasks.c: Include
  <diagnostic.h>, not <diagnostic-core.h>, since the latter is missing
  in 4.5.

gcc-plugin/src/c-expr.y

@@ -42,7 +42,7 @@
   # include <c-pragma.h>
   #endif
 
-  #include <diagnostic-core.h>
+  #include <diagnostic.h>
 
   #if !HAVE_DECL_BUILD_ARRAY_REF
   /* This declaration is missing in GCC 4.6.1.  */

gcc-plugin/src/tasks.c

@@ -31,7 +31,7 @@
 # include <c-common.h>
 #endif
 
-#include <diagnostic-core.h>
+#include <diagnostic.h>
 
 #include <starpu-gcc/tasks.h>
 #include <starpu-gcc/utils.h>