Build: Use FILEPATH type for NASM CMake variable

This causes cmake-gui to to display the proper file chooser dialog
(as opposed to the directory chooser.)

Fixes #40
diff --git a/simd/CMakeLists.txt b/simd/CMakeLists.txt
index 25bd290..b16c467 100755
--- a/simd/CMakeLists.txt
+++ b/simd/CMakeLists.txt
@@ -1,5 +1,5 @@
 if(NOT DEFINED NASM)
-  set(NASM nasm CACHE PATH "Path to NASM/YASM executable")
+  set(NASM nasm CACHE FILEPATH "Path to NASM/YASM executable")
 endif()
 
 if(SIMD_X86_64)