Add explicit io.grpc:grpc-core dependency

Signed-off-by: Chris Bono <chris.bono@gmail.com>

spring-grpc-core/pom.xml

@@ -32,6 +32,10 @@
 			<artifactId>spring-security-web</artifactId>
 			<optional>true</optional>
 		</dependency>
+        <dependency>
+            <groupId>io.grpc</groupId>
+            <artifactId>grpc-core</artifactId>
+        </dependency>
 		<dependency>
 			<groupId>org.springframework.security</groupId>
 			<artifactId>spring-security-oauth2-resource-server</artifactId>