Makefile: Use `:=` instead of `=` on `SOURCE_FILES`

MadScientist (https://stackoverflow.com/users/939557/madscientist) on
Stack Overflow says:

> Also you should always use := not = for shell (and wildcard, for that
> matter) for performance reasons.

(https://stackoverflow.com/questions/26694249/makefiles-using-wildcard-vs-find-for-specifying-source-files/26694693#26694693)

1 files changed, 1 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index 611bc91..cc07bd4 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-SOURCE_FILES = $(shell find src -type f -name '*.rs')
+SOURCE_FILES := $(shell find src -type f -name '*.rs')
 
 LIB := target/debug/libdome_key_map.a