gollvm: add libgmp/mpfr/mpc dependencies for the driver

The driver needs headers gmp.h, etc. to be installed.

Change-Id: If0c047593645831e3512a10471904a382c156d77
Reviewed-on: https://go-review.googlesource.com/116762
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/driver/CMakeLists.txt b/driver/CMakeLists.txt
index 5e5da98..3dc7fd4 100644
--- a/driver/CMakeLists.txt
+++ b/driver/CMakeLists.txt
@@ -37,3 +37,6 @@
   GollvmDriverOptions
   )
 
+# Add dependencies for these libs because it needs the
+# headers to be installed.
+add_dependencies(LLVMDriverUtils libmpfr libmpc libgmp)