gcc: Add missing `-I' CPP flag.

* gcc-plugin/src/Makefile.am (AM_CPPFLAGS): Add
  `-I$(top_srcdir)/include'.  Thanks to Olivier Aumage.

gcc-plugin/src/Makefile.am

@@ -19,5 +19,8 @@ pkglib_LTLIBRARIES = starpu.la
 
 starpu_la_SOURCES = starpu.c c-expr.y
 
-AM_CPPFLAGS = -I$(GCC_PLUGIN_INCLUDE_DIR) -Wall -DYYERROR_VERBOSE=1
+AM_CPPFLAGS =						\
+  -I$(top_srcdir)/include				\
+  -I$(GCC_PLUGIN_INCLUDE_DIR) -Wall -DYYERROR_VERBOSE=1
+
 AM_LDFLAGS = -module