compat: Conditionalize the include of <string>

It is only needed on older protobuf versions where absl::string_view is
not being used.

protoc-gen-c/compat.h

@@ -28,8 +28,6 @@
 #ifndef PROTOBUF_C_PROTOC_GEN_C_COMPAT_H__
 #define PROTOBUF_C_PROTOC_GEN_C_COMPAT_H__
 
-#include <string>
-
 #if GOOGLE_PROTOBUF_VERSION >= 4022000
 # define GOOGLE_ARRAYSIZE	ABSL_ARRAYSIZE
 # define GOOGLE_CHECK_EQ	ABSL_CHECK_EQ
@@ -39,6 +37,8 @@
 
 #if GOOGLE_PROTOBUF_VERSION >= 6030000
 # include <absl/strings/string_view.h>
+#else
+# include <string>
 #endif
 
 namespace protobuf_c {