Fix typo in RPI cross-compiler variable name (#5295)

src/Makefile

@@ -283,7 +283,7 @@ ifeq ($(TARGET_PLATFORM),PLATFORM_DESKTOP_GLFW)
     endif
 endif
 ifeq ($(TARGET_PLATFORM),PLATFORM_DRM)
-    ifeq ($(USE_RPI_CROSS_COMPILER),TRUE)
+    ifeq ($(USE_RPI_CROSSCOMPILER),TRUE)
         # Define RPI cross-compiler
         #CC = armv6j-hardfloat-linux-gnueabi-gcc
         CC = $(RPI_TOOLCHAIN)/bin/$(RPI_TOOLCHAIN_NAME)-gcc