Revert "cleanup: fix grep warning about unnecessary backslash"

Apparently my local `make` interprets this differently to `make` on the
GitHub runner. Different versions maybe? Not sure. Anyway, it broke the
build.

This reverts commit ab96085.

Makefile

@@ -37,7 +37,7 @@ LDFLAGS ?=
 
 SHARED_LDFLAGS ?= -shared
 
-VERSION := $(shell grep '#define.*LIB_JSONNET_VERSION' include/libjsonnet.h | head -n 1 | cut -f 2 -d '"' | sed 's/^v//g' )
+VERSION := $(shell grep '\#define.*LIB_JSONNET_VERSION' include/libjsonnet.h | head -n 1 | cut -f 2 -d '"' | sed 's/^v//g' )
 SOVERSION = 0
 
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