libflash: Set SKIBOOT_VERSION for host-libflash

The recent change to install host-pflash can cause the build to fail
since PFLASH_VERSION is not set:

version.c:1:2: error: #error You need to set PFLASH_VERSION environment variable
 #error You need to set PFLASH_VERSION environment variable
  ^
rules.mk:39: recipe for target 'version.o' failed

PFLASH_VERSION is set from SKIBOOT_VERSION, add it to the build command
as we do for normal libflash build command.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
diff --git a/openpower/package/libflash/libflash.mk b/openpower/package/libflash/libflash.mk
index a409b46..c1c1a15 100644
--- a/openpower/package/libflash/libflash.mk
+++ b/openpower/package/libflash/libflash.mk
@@ -30,7 +30,8 @@
 endef
 
 define HOST_LIBFLASH_BUILD_CMDS
-    $(HOST_MAKE_ENV) $(MAKE) -C $(@D)/external/pflash
+    $(HOST_MAKE_ENV) SKIBOOT_VERSION=$(LIBFLASH_VERSION) \
+	    $(MAKE) -C $(@D)/external/pflash
 endef
 
 define LIBFLASH_INSTALL_STAGING_CMDS