azureblob: enable on freebsd, netbsd, openbsd

At some point the SDK was fixed on these architectures, so re-enable
building the azure blob backend for them.

backend/azureblob/azureblob.go

@@ -1,6 +1,6 @@
 // Package azureblob provides an interface to the Microsoft Azure blob object storage system
 
-// +build !freebsd,!netbsd,!openbsd,!plan9,!solaris,go1.8
+// +build !plan9,!solaris,go1.8
 
 package azureblob
 

backend/azureblob/azureblob_internal_test.go

@@ -1,4 +1,4 @@
-// +build !freebsd,!netbsd,!openbsd,!plan9,!solaris,go1.8
+// +build !plan9,!solaris,go1.8
 
 package azureblob
 

backend/azureblob/azureblob_test.go

@@ -1,6 +1,6 @@
 // Test AzureBlob filesystem interface
 
-// +build !freebsd,!netbsd,!openbsd,!plan9,!solaris,go1.8
+// +build !plan9,!solaris,go1.8
 
 package azureblob
 

backend/azureblob/azureblob_unsupported.go

@@ -1,6 +1,6 @@
 // Build for azureblob for unsupported platforms to stop go complaining
 // about "no buildable Go source files "
 
-// +build freebsd netbsd openbsd plan9 solaris !go1.8
+// +build plan9 solaris !go1.8
 
 package azureblob