Create linker_config_srcs for autogenerated product partition

This will be used to create /product/etc/linker.config.pb. Since
`PRODUCT_PRODUCT_LINKER_CONFIG_FRAGMENTS` are relative to workspace top,
a filegroup soong module will be dynamically created.

Test: checked that /product/etc/linker.config.pb is present in autogenerated
product.img file list (with next CL in stack)

Bug: 376515221
Change-Id: If1cb0d27f79af76c8e5780ead12595518b154b9e
diff --git a/android/variable.go b/android/variable.go
index 548a762..a386473 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -596,9 +596,10 @@
 
 	BoardAvbEnable bool `json:",omitempty"`
 
-	ProductPackages        []string `json:",omitempty"`
-	ProductPackagesDebug   []string `json:",omitempty"`
-	VendorLinkerConfigSrcs []string `json:",omitempty"`
+	ProductPackages         []string `json:",omitempty"`
+	ProductPackagesDebug    []string `json:",omitempty"`
+	VendorLinkerConfigSrcs  []string `json:",omitempty"`
+	ProductLinkerConfigSrcs []string `json:",omitempty"`
 
 	ProductCopyFiles map[string]string `json:",omitempty"`
 }