ARROW-2424: [Rust] Fix build - add missing import
ARROW-2424: [Rust] Fix build - add missing import

@pitrou recent merges broke the build somehow - this fixes it

Author: Andy Grove <andygrove73@gmail.com>

Closes #1864 from andygrove/fix_build and squashes the following commits:

2f87c70 <Andy Grove> Fix build - add missing import

wagavulin/arrow,pcmoritz/arrow,icexelloss/arrow,itaiin/arrow,wagavulin/arrow,xhochy/arrow,cpcloud/arrow,xhochy/arrow,kou/arrow,pcmoritz/arrow,pcmoritz/arrow,apache/arrow,renesugar/arrow,xhochy/arrow,cpcloud/arrow,itaiin/arrow,tebeka/arrow,wagavulin/arrow,kou/arrow,apache/arrow,majetideepak/arrow,tebeka/arrow,xhochy/arrow,pcmoritz/arrow,wagavulin/arrow,renesugar/arrow,wesm/arrow,icexelloss/arrow,renesugar/arrow,icexelloss/arrow,renesugar/arrow,itaiin/arrow,cpcloud/arrow,pcmoritz/arrow,cpcloud/arrow,pcmoritz/arrow,apache/arrow,wesm/arrow,renesugar/arrow,majetideepak/arrow,xhochy/arrow,xhochy/arrow,pcmoritz/arrow,wagavulin/arrow,itaiin/arrow,xhochy/arrow,kou/arrow,wagavulin/arrow,cpcloud/arrow,majetideepak/arrow,xhochy/arrow,apache/arrow,pcmoritz/arrow,pcmoritz/arrow,itaiin/arrow,cpcloud/arrow,laurentgo/arrow,itaiin/arrow,tebeka/arrow,tebeka/arrow,icexelloss/arrow,majetideepak/arrow,icexelloss/arrow,majetideepak/arrow,itaiin/arrow,renesugar/arrow,kou/arrow,itaiin/arrow,pcmoritz/arrow,majetideepak/arrow,kou/arrow,tebeka/arrow,laurentgo/arrow,pcmoritz/arrow,cpcloud/arrow,pcmoritz/arrow,itaiin/arrow,wesm/arrow,apache/arrow,icexelloss/arrow,laurentgo/arrow,itaiin/arrow,majetideepak/arrow,icexelloss/arrow,laurentgo/arrow,wesm/arrow,wesm/arrow,kou/arrow,majetideepak/arrow,wesm/arrow,wesm/arrow,icexelloss/arrow,itaiin/arrow,xhochy/arrow,itaiin/arrow,wagavulin/arrow,wagavulin/arrow,renesugar/arrow,renesugar/arrow,apache/arrow,icexelloss/arrow,cpcloud/arrow,wagavulin/arrow,cpcloud/arrow,icexelloss/arrow,wesm/arrow,wesm/arrow,apache/arrow,wagavulin/arrow,wesm/arrow,renesugar/arrow,xhochy/arrow,tebeka/arrow,xhochy/arrow,icexelloss/arrow,itaiin/arrow,tebeka/arrow,majetideepak/arrow,laurentgo/arrow,majetideepak/arrow,majetideepak/arrow,wagavulin/arrow,cpcloud/arrow,icexelloss/arrow,kou/arrow,kou/arrow,apache/arrow,wesm/arrow,apache/arrow,kou/arrow,renesugar/arrow,renesugar/arrow,pcmoritz/arrow,apache/arrow,cpcloud/arrow,itaiin/arrow,pcmoritz/arrow,wesm/arrow,wesm/arrow,apache/arrow,icexelloss/arrow,laurentgo/arrow,laurentgo/arrow,laurentgo/arrow,xhochy/arrow,wesm/arrow,renesugar/arrow,renesugar/arrow,icexelloss/arrow,apache/arrow,cpcloud/arrow,apache/arrow,xhochy/arrow,tebeka/arrow,laurentgo/arrow,apache/arrow,wagavulin/arrow,kou/arrow,cpcloud/arrow,xhochy/arrow,laurentgo/arrow,majetideepak/arrow,itaiin/arrow,renesugar/arrow,laurentgo/arrow,kou/arrow,pcmoritz/arrow,tebeka/arrow,laurentgo/arrow,majetideepak/arrow,laurentgo/arrow,laurentgo/arrow,kou/arrow,majetideepak/arrow,cpcloud/arrow,renesugar/arrow,tebeka/arrow,kou/arrow,kou/arrow,majetideepak/arrow,laurentgo/arrow,xhochy/arrow