deps: explicitly define grpc-gcp version

Author: olavloite

pom.xml

@@ -110,6 +110,11 @@
       <groupId>com.google.cloud</groupId>
       <artifactId>google-cloud-spanner</artifactId>
     </dependency>
+    <dependency>
+      <groupId>com.google.cloud</groupId>
+      <artifactId>grpc-gcp</artifactId>
+      <version>1.9.0</version>
+    </dependency>
     <!-- TODO: Remove grpc-alts from here once it has been removed as a runtime dependency from java-spanner -->
     <dependency>
       <groupId>io.grpc</groupId>