drivers/of: include export.h for EXPORT_SYMBOL_GPL/THIS_MODULE as required

For files that were getting these macros via the implicit presence
of module.h being everywhere.

With contributions from Stephen Rothwell <[e-mail]>.

Signed-off-by: Paul Gortmaker <[e-mail]>

drivers/of/of_net.c

···
 #include <linux/kernel.h>
 #include <linux/of_net.h>
 #include <linux/phy.h>
+#include <linux/export.h>
 
 /**
  * It maps 'enum phy_interface_t' found in include/linux/phy.h

drivers/of/of_pci_irq.c

···
 #include <linux/kernel.h>
 #include <linux/of_pci.h>
 #include <linux/of_irq.h>
+#include <linux/export.h>
 #include <asm/prom.h>
 
 /**