modules/allwinner: add nvmem_if.h to SRCS

Ensure the header is generated; it is a dependency for these drivers.
This fixes standalone module builds and riscv LINT.

Reported by:	zlei
Sponsored by:	The FreeBSD Foundation
Fixes:	5522519731 ("modules: enable allwinner kmods on riscv")

sys/modules/allwinner/aw_sid/Makefile

@@ -7,6 +7,7 @@ SRCS+=	\
 	bus_if.h \
 	clknode_if.h \
 	device_if.h \
-	ofw_bus_if.h \
+	nvmem_if.h \
+	ofw_bus_if.h
 
 .include <bsd.kmod.mk>

sys/modules/allwinner/aw_thermal/Makefile

@@ -7,6 +7,7 @@ SRCS+=	\
 	bus_if.h \
 	clknode_if.h \
 	device_if.h \
-	ofw_bus_if.h \
+	nvmem_if.h \
+	ofw_bus_if.h
 
 .include <bsd.kmod.mk>